fix: fix the v-for attribute value (vuejs#1561)

Author: chenpeng991105

src/examples/src/cells/App/template.html

@@ -8,7 +8,7 @@
   <tbody>
     <tr v-for="i in cells[0].length">
       <th>{{ i - 1 }}</th>
-      <td v-for="c, j in cols">
+      <td v-for="(c, j) in cols">
         <Cell :r="i - 1" :c="j"></Cell>
       </td>
     </tr>