Properly quote python version number in workflow definition

.github/workflows/docs.yml

@@ -26,7 +26,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.10
+          python-version: '3.10'
       - name: Install dependencies
         run: pip install -r requirements.txt
         working-directory: ./docs