publish-your-site:fix add missing step id

The step id is referenced by the environment, which collects the
output page_url from the deploy_pages action. This is then displayed
in the GitHub Actions UI.

Author: alexvoss

docs/publish-your-site.md

@@ -55,6 +55,7 @@ jobs:
         with:
           path: site
       - uses: actions/deploy-pages@v4
+        id: deployment
 ```
 
 1.  At the moment, we do not recommend using caches on CI systems as the caching