message: docs: remove index key
diff: @@ -140,7 +140,7 @@ Use title slot to custom tab title.
  ```html
  <van-tabs v-model:active="active">
  - <van-tab v-for="index in 2" :key="index">
  + <van-tab v-for="index in 2">
  <template #title> <van-icon name="more-o" />tab </template>
  content {{ index }}
  </van-tab>
  