Bump peaceiris/actions-gh-pages from 3.9.3 to 4.0.0 (#18)

Author: dependabot[bot]

.github/workflows/build.yml

@@ -68,7 +68,7 @@ jobs:
       - name: Deploy to gh-pages
         if: success() && github.event_name != 'pull_request'
         # Don't use tags: https://julienrenaux.fr/2019/12/20/github-actions-security-risk/
-        uses: peaceiris/actions-gh-pages@373f7f263a76c20808c831209c920827a82a2847
+        uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: _build/html