Update gh pages deploy workflow

Author: marko-radosavljevic

.github/workflows/deploy-gh-pages.yaml

@@ -22,7 +22,7 @@ jobs:
 
       - name: Deploy
         uses: peaceiris/actions-gh-pages@v3
-        if: ${{ github.ref == 'refs/heads/main' }}
+        if: ${{ github.ref == 'refs/heads/master' }}
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./book