Fix publish-gem workflow

Author: pcriv

.github/workflows/publish-gem.yml

@@ -12,7 +12,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
       with:
-        ref: ${GITHUB_REF}
+        ref: "${{ github.event.release.tag_name }}"
     - name: Set up Ruby 2.6
       uses: actions/setup-ruby@v1
       with: