changelog: Fix 0.3.0 link

Author: oblique

CHANGELOG.md

@@ -70,7 +70,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [First release](https://docs.rs/async-tftp/0.1.0)
 
 
-[unreleased]: https://github.com/oblique/async-tftp-rs/compare/0.2.0...HEAD
+[unreleased]: https://github.com/oblique/async-tftp-rs/compare/0.3.0...HEAD
+[0.3.0]: https://github.com/oblique/async-tftp-rs/compare/0.2.0...0.3.0
 [0.2.0]: https://github.com/oblique/async-tftp-rs/compare/0.1.3...0.2.0
 [0.1.3]: https://github.com/oblique/async-tftp-rs/compare/0.1.2...0.1.3
 [0.1.2]: https://github.com/oblique/async-tftp-rs/compare/0.1.1...0.1.2