Fix logging typo

When launching AB3 from terminal using java -jar, the start of logging
says 'Starting AddressBook' while the end says 'Stopping Address Book'.

Let's fix this inconsistency by changing it to 'Stopping AddressBook' in
MainApp.java

src/main/java/seedu/address/MainApp.java

@@ -176,7 +176,7 @@ public void start(Stage primaryStage) {
 
     @Override
     public void stop() {
-        logger.info("============================ [ Stopping Address Book ] =============================");
+        logger.info("============================ [ Stopping AddressBook ] =============================");
         try {
             storage.saveUserPrefs(model.getUserPrefs());
         } catch (IOException e) {