Update CHANGELOG for version 1.8.1, reflecting new release links and version updates

Author: alex-omophub

CHANGELOG.md

@@ -275,7 +275,9 @@ and are not shipped in the wheel/sdist.
 - Full type hints and PEP 561 compliance
 - HTTP/2 support via httpx
 
-[Unreleased]: https://github.com/omopHub/omophub-python/compare/v1.7.1...HEAD
+[Unreleased]: https://github.com/omopHub/omophub-python/compare/v1.8.1...HEAD
+[1.8.1]: https://github.com/omopHub/omophub-python/compare/v1.8.0...v1.8.1
+[1.8.0]: https://github.com/omopHub/omophub-python/compare/v1.7.1...v1.8.0
 [1.7.1]: https://github.com/omopHub/omophub-python/compare/v1.7.0...v1.7.1
 [1.7.0]: https://github.com/omopHub/omophub-python/compare/v1.6.0...v1.7.0
 [1.6.0]: https://github.com/omopHub/omophub-python/compare/v1.5.1...v1.6.0