Upgrade action

Author: tvdijen

.github/workflows/mk_docs.yml

@@ -30,7 +30,7 @@ jobs:
          python mk_docs.py 2>&1
 
     - name: Deploy
-      uses: peaceiris/actions-gh-pages@v3
+      uses: peaceiris/actions-gh-pages@v4
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         publish_dir: ./_site