Update CHANGELOG links

Author: kddnewton

CHANGELOG.md

@@ -276,8 +276,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 
 - 🎉 Initial release! 🎉
 
-[unreleased]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.1.0...HEAD
-[2.1.0]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.8.0...v2.1.0
+[unreleased]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.9.0...HEAD
+[2.9.0]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.8.0...v2.9.0
 [2.8.0]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.7.1...v2.8.0
 [2.7.1]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.7.0...v2.7.1
 [2.7.0]: https://github.com/ruby-syntax-tree/syntax_tree/compare/v2.6.0...v2.7.0