changelog: Fix link

Author: real-or-random

CHANGELOG.md

@@ -67,6 +67,7 @@ The number was given by the build system since the introduction of autotools in
 Therefore, this version number does not uniquely identify a set of source files.
 
 [unreleased]: https://github.com/bitcoin-core/secp256k1/compare/v0.3.0...HEAD
+[0.3.1]: https://github.com/bitcoin-core/secp256k1/compare/v0.3.0...v0.3.1
 [0.3.0]: https://github.com/bitcoin-core/secp256k1/compare/v0.2.0...v0.3.0
 [0.2.0]: https://github.com/bitcoin-core/secp256k1/compare/423b6d19d373f1224fd671a982584d7e7900bc93..v0.2.0
 [0.1.0]: https://github.com/bitcoin-core/secp256k1/commit/423b6d19d373f1224fd671a982584d7e7900bc93