Fix GitHub Actions error: A branch or tag with the name '3' could not be found.

.github/workflows/publish_api_documentation.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
       with:
-        ref: 3.0
+        ref: "3.0"
     - uses: actions/setup-python@v2
       with:
         python-version: 3.6