fix pre-release flag

Author: Stephan Dilly

.github/workflows/cd.yml

@@ -71,7 +71,7 @@ jobs:
       uses: softprops/action-gh-release@v1
       with:
         body: ${{ steps.release_notes.outputs.release_notes }}
-        prerelease: contains(github.ref, '-')
+        prerelease: ${{ contains(github.ref, '-') }}
         files: |
           ./release/*.tar.gz
           ./release/*.zip