docs(components): use fallbackTag in ClientOnly examples (#6587)

Author: HomWang

docs/content/2.guide/3.directory-structure/4.components.md

@@ -164,7 +164,8 @@ Use a slot as fallback until `<ClientOnly>` is mounted on client side.
 <template>
   <div>
     <Sidebar />
-    <ClientOnly>
+    <!-- This renders the "span" element on the server side -->
+    <ClientOnly fallbackTag="span">
       <!-- this component will only be rendered on client side -->
       <Comments />
       <template #fallback>