Fix links in changelog

Author: LukasKalbertodt

CHANGELOG.md

@@ -76,7 +76,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Everything.
 
 
-[Unreleased]: https://github.com/LukasKalbertodt/atomig/compare/v0.3.3...HEAD
+[Unreleased]: https://github.com/LukasKalbertodt/atomig/compare/v0.4.0...HEAD
+[0.4.0]: https://github.com/LukasKalbertodt/atomig/compare/v0.3.3...v0.4.0
 [0.3.3]: https://github.com/LukasKalbertodt/atomig/compare/v0.3.2...v0.3.3
 [0.3.2]: https://github.com/LukasKalbertodt/atomig/compare/v0.3.1...v0.3.2
 [0.3.1]: https://github.com/LukasKalbertodt/atomig/compare/v0.3.0...v0.3.1