use more recent version of action to publish

(looks like subdirectory html/_images is not copied)

Author: fangohr

.github/workflows/deploy.yml

@@ -54,7 +54,7 @@ jobs:
 
     # Deploy the book's HTML to gh-pages branch
     - name: GitHub Pages action
-      uses: peaceiris/actions-gh-pages@bbdfb200618d235585ad98e965f4aafc39b4c501
+      uses: peaceiris/actions-gh-pages@v3
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
         publish_dir: ./book/_build/html