docs: Improve custom branch clarity (#761)

README.md

@@ -163,6 +163,9 @@ jobs:
 
       - name: Deploy
         uses: peaceiris/actions-gh-pages@v3
+        # If you're changing the branch from main, 
+        # also change the `main` in `refs/heads/main` 
+        # below accordingly.
         if: ${{ github.ref == 'refs/heads/main' }}
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}