fix: revise the migration guide for $children, especially the example (vuejs#688)

Author: skirtles-code

Diff for: src/guide/migration/children.md

@@ -7,32 +7,34 @@ badges:
 
 ## Overview
 
-`$children` instance property removed from Vue 3.0 and no longer supported.
+The `$children` instance property has been removed from Vue 3.0 and is no longer supported.
 
 ## 2.x Syntax
 
 In 2.x, developers could access direct child components of the current instance with `this.$children`:
 
-```html
-<div ref="app" id="app">
-  <img alt="Vue logo" src="./assets/logo.png" width="25%" />
-  <my-button>Change logo</my-button>
-</div>
-```
+```vue
+<template>
+  <div>
+    <img alt="Vue logo" src="./assets/logo.png">
+    <my-button>Change logo</my-button>
+  </div>
+</template>
+
+<script>
+import MyButton from './MyButton'
 
-```js
 export default {
-  name: "App",
   components: {
-    MyButton,
+    MyButton
   },
   mounted() {
-    console.log(this.$children); // [VueComponent]
-  },
-};
+    console.log(this.$children) // [VueComponent]
+  }
+}
 </script>
 ```
 
 ## 3.x Update
 
-In 3.x, `$children` property is removed and no longer supported. Instead, if you need to access a child component instance, we recommend using [$refs](/guide/component-template-refs.html#template-refs).
+In 3.x, the `$children` property is removed and no longer supported. Instead, if you need to access a child component instance, we recommend using [$refs](/guide/component-template-refs.html#template-refs).