Use keyed loop (vuejs#1767)

Per https://vuejs.org/v2/style-guide/#Keyed-v-for-essential the loop should be keyed, I feel this should be reflected in the guide.

Author: hkraal

src/v2/guide/index.md

@@ -151,7 +151,9 @@ There are quite a few other directives, each with its own special functionality.
 ``` html
 <div id="app-4">
   <ol>
-    <li v-for="todo in todos">
+    <li v-for="todo in todos"
+      :key="todo.id"
+    >
       {{ todo.text }}
     </li>
   </ol>
@@ -162,9 +164,9 @@ var app4 = new Vue({
   el: '#app-4',
   data: {
     todos: [
-      { text: 'Learn JavaScript' },
-      { text: 'Learn Vue' },
-      { text: 'Build something awesome' }
+      { id: 1, text: 'Learn JavaScript' },
+      { id: 2, text: 'Learn Vue' },
+      { id: 3, text: 'Build something awesome' }
     ]
   }
 })