Tag: fix a doc issue, fixed #2611 (#2613)

examples/docs/en-US/tag.md

@@ -115,6 +115,7 @@ You can use the `close` event to add and remove tag dynamically.
 :::demo
 ```html
 <el-tag
+  :key="tag"
   v-for="tag in dynamicTags"
   :closable="true"
   :close-transition="false"

examples/docs/zh-CN/tag.md

@@ -115,6 +115,7 @@
 :::demo
 ```html
 <el-tag
+  :key="tag"
   v-for="tag in dynamicTags"
   :closable="true"
   :close-transition="false"