✏️ fix link in upgrade guide

Signed-off-by: burgholzer <burgholzer@me.com>

Author: burgholzer

UPGRADING.md

@@ -185,7 +185,7 @@ It also requires the `uv` library version 0.5.20 or higher.
 <!-- Version links -->
 
 [unreleased]: https://github.com/munich-quantum-toolkit/core/compare/v3.4.0...HEAD
-[3.4.0]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.4.0
+[3.4.0]: https://github.com/munich-quantum-toolkit/core/compare/v3.3.0...v3.4.0
 [3.3.0]: https://github.com/munich-quantum-toolkit/core/compare/v3.2.0...v3.3.0
 [3.2.0]: https://github.com/munich-quantum-toolkit/core/compare/v3.1.0...v3.2.0
 [3.1.0]: https://github.com/munich-quantum-toolkit/core/compare/v3.0.0...v3.1.0