Update step4-content.md (#154)

Co-authored-by: Zyie <[email protected]>

src/tutorials/v8.0.0/chooChooTrain/step4/step4-content.md

@@ -71,8 +71,8 @@ return graphics;
 With the `createMountainGroup()` helper function, we can then create 2 instances of the mountain group and offset one of them off the screen to the right.
 
 ```javascript
-const group1 = createMountainGroup();
-const group2 = createMountainGroup();
+const group1 = createMountainGroup(app);
+const group2 = createMountainGroup(app);
 
 group2.x = app.screen.width;
 app.stage.addChild(group1, group2);
@@ -101,4 +101,4 @@ app.ticker.add((time) =>
         group2.x += app.screen.width * 2;
     }
 });
-```
+```