Update autogenerated notice to refer to specific ruamel.yaml version

.github/workflows/pr-checks.yml

@@ -52,6 +52,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
+          # When updating this, update the autogenerated code header in `sync.py` too.
           pip install ruamel.yaml==0.17.31
 
       # Ensure the generated PR check workflows are up to date.