Revert "Fix typo: 'is' => 'id'" (vuejs#758)

src/v2/guide/list.md

@@ -244,7 +244,7 @@ Here's a complete example of a simple todo list:
   >
   <ul>
     <li
-      id="todo-item"
+      is="todo-item"
       v-for="(todo, index) in todos"
       v-bind:title="todo"
       v-on:remove="todos.splice(index, 1)"