message: Fix links in changelog.
diff: @@ -230,7 +230,7 @@ more details
  - Initial release on Crates.io.
  -[Unreleased]: https://github.com/bheisler/criterion.rs/compare/0.2.7...HEAD
  +[Unreleased]: https://github.com/bheisler/criterion.rs/compare/0.2.8...HEAD
  [0.1.1]: https://github.com/bheisler/criterion.rs/compare/0.1.0...0.1.1
  [0.1.2]: https://github.com/bheisler/criterion.rs/compare/0.1.1...0.1.2
  [0.2.0]: https://github.com/bheisler/criterion.rs/compare/0.1.2...0.2.0
  @@ -240,4 +240,5 @@ more details
  [0.2.4]: https://github.com/bheisler/criterion.rs/compare/0.2.3...0.2.4
  [0.2.5]: https://github.com/bheisler/criterion.rs/compare/0.2.4...0.2.5
  [0.2.6]: https://github.com/bheisler/criterion.rs/compare/0.2.5...0.2.6
  -[0.2.7]: https://github.com/bheisler/criterion.rs/compare/0.2.5...0.2.7
  +[0.2.7]: https://github.com/bheisler/criterion.rs/compare/0.2.6...0.2.7
  +[0.2.8]: https://github.com/bheisler/criterion.rs/compare/0.2.7...0.2.8
  