docs: switch v-if with class for consistency (#2750)

Author: Patrick Sachs

Diff for: docs/guide/advanced/data-fetching.md

@@ -17,7 +17,7 @@ Let's assume we have a `Post` component that needs to fetch the data for a post
 ``` html
 <template>
   <div class="post">
-    <div class="loading" v-if="loading">
+    <div v-if="loading" class="loading">
       Loading...
     </div>