Fix broken React Router v6 migration guide link

The link was returning 404. Updated from the incorrect path:
- /en/main/upgrading/v5 (broken)
to the working path:
- /docs/en/v6/upgrading/v5 (verified working)

This link was added in commit 391281a when updating React Router
documentation to v6. The URL structure changed on reactrouter.com.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ihabadham

docs/building-features/react-router.md

@@ -179,5 +179,5 @@ For a practical example of route organization, see the [routes configuration fil
 ## Additional Resources
 
 - [React Router Official Documentation](https://reactrouter.com/)
-- [React Router v6 Migration Guide](https://reactrouter.com/en/main/upgrading/v5) - If upgrading from v5
+- [React Router v6 Migration Guide](https://reactrouter.com/docs/en/v6/upgrading/v5) - If upgrading from v5
 - [React on Rails Turbo/Turbolinks Guide](./turbolinks.md)