Add missing closing </AppOnly> (#72453)

The MDX was invalid after #72399
causing next-site deploy failures

(I'm not entirely sure if this is where the `</AppOnly>` should go, feel
free to correct that)

Closes PACK-3416

Author: mischnic

Diff for: docs/01-app/01-getting-started/02-project-structure.mdx

@@ -311,3 +311,5 @@ This strategy stores globally shared application code in the root `app` director
   width="1600"
   height="1011"
 />
+
+</AppOnly>