fix 1.2.0 version link in changelog

Author: jakoch

CHANGELOG.md

@@ -65,6 +65,7 @@ The project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html
 
 - dropped the deprecated Python 2
 
-[unreleased]: https://github.com/jakoch/jupyter-devbox/compare/v1.1.0...HEAD
+[unreleased]: https://github.com/jakoch/jupyter-devbox/compare/v1.2.0...HEAD
+[1.2.0]: https://github.com/jakoch/jupyter-devbox/compare/v1.1.0...v1.2.0
 [1.1.0]: https://github.com/jakoch/jupyter-devbox/compare/v1.0.0...v1.1.0
 [1.0.0]: https://github.com/jakoch/jupyter-devbox/releases/tag/v1.0.0