Fix broken README.md UX guide link.

Replace broken link to UX guide with new working link in the README.md
file.

README.md

@@ -84,7 +84,7 @@ For more information please see the [architecture documentation](https://docs.gi
 ## UX design
-Please adhere to the [UX Guide](doc/development/ux_guide/readme.md) when creating designs and implementing code.
+Please adhere to the [UX Guide](doc/development/ux_guide/index.md) when creating designs and implementing code.
 ## Third-party applications

changelogs/unreleased/readme-link-fix.yml

+---
+title: Fix broken README.md UX guide link.
+merge_request: 
+author: