Update reactivity.md

Author: WangNingning1994

src/v2/guide/reactivity.md

@@ -90,7 +90,7 @@ vm.items.splice(indexOfItem, 1, newValue)
 You can also use the [`vm.$set`](/v2/api/#vm-set) instance method, which is an alias for the global `Vue.set`:
 
 ``` js
-vm.$set(vm.items, indexOfItem, newValue)
+this.$set(this.items, indexOfItem, newValue)
 ```
 
 To deal with caveat 2, you can use `splice`: