Add missing changelog link

Author: stephane-caron

CHANGELOG.md

@@ -522,7 +522,8 @@ This initial release includes 33 robot descriptions:
 - Contributing instructions
 - This changelog
 
-[unreleased]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.20.0...HEAD
+[unreleased]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.21.0...HEAD
+[1.21.0]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.20.0...v1.21.0
 [1.20.0]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.19.0...v1.20.0
 [1.19.0]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.18.0...v1.19.0
 [1.18.0]: https://github.com/robot-descriptions/robot_descriptions.py/compare/v1.17.0...v1.18.0