Changed ES7 to ES2016 as per comment (#1839)

* Changed ES7 to ES2016 as per comment

Changed the ES7 nickname to ES2016 as per a comment request. Also added `Vue.component` so syntax highlighter will pick up on the code.

* Removed line in favour of explicit js indicator

as per @phanan suggestion:
I removed the `Vue.Component` code line in favour of an explicit js indicator.

* Fixed js indicator

In previous commit I removed the js indicator from the component.

Author: hitautodestruct

src/v2/guide/reactivity.md

@@ -119,10 +119,9 @@ Vue.component('example', {
 })
 ```
 
-Since `$nextTick()` returns a promise, you can achieve the same as the above using the new [ES7 async/await](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) syntax:
+Since `$nextTick()` returns a promise, you can achieve the same as the above using the new [ES2016 async/await](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function) syntax:
 
-```
-...
+``` js
   methods: {
     async updateMessage: function () {
       this.message = 'updated'
@@ -131,5 +130,4 @@ Since `$nextTick()` returns a promise, you can achieve the same as the above usi
       console.log(this.$el.textContent) // => 'updated'
     }
   }
-...
 ```