Update list.md (#1429)

* Update list.md

* Update list.md

link `vm.$set` to the API doc

Author: 01abhishekjain

src/v2/guide/list.md

@@ -268,21 +268,39 @@ Due to limitations in JavaScript, Vue **cannot** detect the following changes to
 1. When you directly set an item with the index, e.g. `vm.items[indexOfItem] = newValue`
 2. When you modify the length of the array, e.g. `vm.items.length = newLength`
 
+For example:
+
+``` js
+var vm = new Vue({
+  data: {
+    items: ['a', 'b', 'c']
+  }
+})
+vm.items[1] = 'x' // is NOT reactive
+vm.items.length = 2 // is NOT reactive
+```
+
 To overcome caveat 1, both of the following will accomplish the same as `vm.items[indexOfItem] = newValue`, but will also trigger state updates in the reactivity system:
 
 ``` js
 // Vue.set
-Vue.set(example1.items, indexOfItem, newValue)
+Vue.set(vm.items, indexOfItem, newValue)
 ```
 ``` js
 // Array.prototype.splice
-example1.items.splice(indexOfItem, 1, newValue)
+vm.items.splice(indexOfItem, 1, newValue)
+```
+
+You can also use the [`vm.$set`](https://vuejs.org/v2/api/#vm-set) instance method, which is an alias for the global `Vue.set`:
+
+``` js
+vm.$set(vm.items, indexOfItem, newValue)
 ```
 
 To deal with caveat 2, you can use `splice`:
 
 ``` js
-example1.items.splice(newLength)
+vm.items.splice(newLength)
 ```
 
 ## Object Change Detection Caveats