update github action to fix cname dropping

Author: robguthrie

.github/workflows/mdbook.yml

@@ -22,8 +22,9 @@ jobs:
       - run: mdbook build
 
       - name: Deploy
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         if: ${{ github.ref == 'refs/heads/main' }}
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./book
+          cname: handbook.enspiral.com