Fix link to index.md in development README.md

[ci skip]

doc/development/README.md

@@ -14,7 +14,7 @@
   contributing to documentation.
 - [SQL Migration Style Guide](migration_style_guide.md) for creating safe SQL migrations
 - [Testing standards and style guidelines](testing.md)
-- [UX guide](ux_guide/README.md) for building GitLab with existing CSS styles and elements
+- [UX guide](ux_guide/index.md) for building GitLab with existing CSS styles and elements
 - [Frontend guidelines](frontend.md)
 - [SQL guidelines](sql.md) for working with SQL queries
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers