Fix MDX syntax error by adding missing opening code fence

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

apps/site/content/docs/guide/index.mdx

@@ -307,6 +307,7 @@ curl -X POST http://localhost:3000/api/data/contacts \
 
 > **Note**: The UI components have been moved to a separate project and are no longer part of this monorepo. They can be integrated with ObjectOS through the HTTP API layer (`@objectos/server`) which provides REST endpoints for metadata and data access.
 
+```jsx
 function ContactDetail({ contactId }) {
   return (
     <div>