Fix broken changelog links (#63)

Author: f3ath

CHANGELOG.md

@@ -93,7 +93,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Client: fetch resources, collections, related resources and relationships
 
-[Unreleased]: https://github.com/f3ath/json-api-dart/compare/2.0.3...HEAD
+[Unreleased]: https://github.com/f3ath/json-api-dart/compare/2.1.0...HEAD
+[2.1.0]: https://github.com/f3ath/json-api-dart/compare/2.0.3...2.1.0
 [2.0.3]: https://github.com/f3ath/json-api-dart/compare/2.0.2...2.0.3
 [2.0.2]: https://github.com/f3ath/json-api-dart/compare/2.0.1...2.0.2
 [2.0.1]: https://github.com/f3ath/json-api-dart/compare/2.0.0...2.0.1