@v4 not @4 for checkout action

Author: petercorke

.github/workflows/main-doc.yml

@@ -20,7 +20,7 @@ jobs:
         python-version: 3.9
     - name: Checkout main branch
       # checkout mvtb repo into workspace
-      uses: actions/checkout@4
+      uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip