docs: update menu template demo

Author: tangjinzhou

components/menu/demo/SubMenu.vue

@@ -1,5 +1,5 @@
 <template functional>
-  <a-sub-menu v-on="listeners" v-bind="data.attrs">
+  <a-sub-menu v-on="listeners" v-bind="data.attrs" :key="data.key">
     <span slot="title"><a-icon type="mail" /><span>{{data.attrs.menuInfo.title}}</span></span>
     <template v-for="item in data.attrs.menuInfo.children">
       <a-menu-item v-if="!item.children" :key="item.key">