result of Observable is not a String, but an Observable (mobxjs#787)

@all-contributors add @vandadnp for doc

README.md

@@ -252,7 +252,7 @@ Runs the reaction immediately and also on any change in the observables used ins
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -277,7 +277,7 @@ the predicate returns a different value. Only the observables inside `predicate(
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -298,7 +298,7 @@ Monitors the observables used inside `predicate()` and runs the `effect()` _when
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));
 

docs/docs/api/reaction.mdx

@@ -25,7 +25,7 @@ Runs the reaction immediately and also on any change in the observables used ins
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -52,7 +52,7 @@ the tracking function returns a different value. Only the observables inside `fn
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -75,7 +75,7 @@ Monitors the observables used inside `predicate()` and runs the `effect()` _when
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));
 

docs/docs/core-concepts.mdx

@@ -170,7 +170,7 @@ Runs the reaction immediately and also on any change in the observables used ins
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -195,7 +195,7 @@ the tracking function returns a different value. Only the observables inside `fn
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -216,7 +216,7 @@ Monitors the observables used inside `predicate()` and runs the `effect()` _when
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));
 

mobx/README.md

@@ -212,7 +212,7 @@ Runs the reaction immediately and also on any change in the observables used ins
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -237,7 +237,7 @@ the `fn()` function returns a different value. Only the observables inside `fn()
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -258,7 +258,7 @@ Monitors the observables used inside `predicate()` and runs the `effect()` _when
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));
 

translation/pt-BR/README.md

@@ -222,7 +222,7 @@ Executa uma Reaction na hora em que é rastreada na função anônima `fn`.
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -246,7 +246,7 @@ Monitora um observável dentro de uma função de `predicate()` e executa o `eff
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -267,7 +267,7 @@ Monitora um observável dentro de uma função de `predicate()` e executa o `eff
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));
 

translation/zh-CN/README.md

@@ -223,7 +223,7 @@ Future<void> loadStuff() async {
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = autorun((_){
   print(greeting.value);
@@ -247,7 +247,7 @@ dispose();
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = reaction((_) => greeting.value, (msg) => print(msg));
 
@@ -268,7 +268,7 @@ dispose();
 ```dart
 import 'package:mobx/mobx.dart';
 
-String greeting = Observable('Hello World');
+final greeting = Observable('Hello World');
 
 final dispose = when((_) => greeting.value == 'Hello MobX', () => print('Someone greeted MobX'));