doc(nyz): indicate doc repo branch name in doc workflow

.github/workflows/doc.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Generate
         run: |
           python -m pip install .
-          git clone https://github.com/opendilab/DI-engine-docs.git
+          git clone -b main https://github.com/opendilab/DI-engine-docs.git
           cd DI-engine-docs
           python -m pip install -r requirements.txt
           make html