Add :key prop on exemple 1 (#2493)

Author: lelibreauquotidien

src/v2/guide/list.md

@@ -13,7 +13,7 @@ We can use the `v-for` directive to render a list of items based on an array. Th
 
 ``` html
 <ul id="example-1">
-  <li v-for="item in items">
+  <li v-for="item in items" :key="item.message">
     {{ item.message }}
   </li>
 </ul>
@@ -35,7 +35,7 @@ Result:
 
 {% raw %}
 <ul id="example-1" class="demo">
-  <li v-for="item in items">
+  <li v-for="item in items" :key="item.message">
     {{item.message}}
   </li>
 </ul>