Add required v-for :key to the the second component list example. Related to PR vuejs#792 (vuejs#801)

Author: johnathankent

src/v2/guide/list.md

@@ -229,7 +229,8 @@ new Vue({ el: '#range' })
 <my-component
   v-for="(item, index) in items"
   v-bind:item="item"
-  v-bind:index="index">
+  v-bind:index="index"
+  v-bind:key="item.id">
 </my-component>
 ```