Transition from Travis CI to GitHub Actions (chapter 3)

1. Update documentation.yml

Author: jointpoints

.github/workflows/documentation.yml

@@ -29,6 +29,6 @@ jobs:
       uses: peaceiris/actions-gh-pages@v3.6.4
       with:
         github_token: ${{ secrets.GITHUB_TOKEN }}
-        publish_dir: "."
+        publish_dir: "./tmp_docs"
         publish_branch: gh-pages