ci(#129): fix changelog url

.github/workflows/ci.yaml

@@ -97,7 +97,7 @@ jobs:
               > release.md
           git tag -l --format="%(contents)" ${{ github.ref_name }} | sed '/^-----BEGIN PGP SIGNATURE-----/,$d' \
               >> release.md
-          echo -e "\n**Full changelog**: https://github.com/jannis-baum/vivify/compare/$(git tag --sort=version:refname | tail -2 | head -1)...${{ github.ref }}" \
+          echo -e "\n**Full changelog**: https://github.com/jannis-baum/vivify/compare/$(git tag --sort=version:refname | tail -2 | head -1)...${{ github.ref_name }}" \
               >> release.md
           echo -e '\n**SHA256 checksums**\\' \
               >> release.md