fix: do not try to deploy mdbook if not main

.github/workflows/mdBook.yml

@@ -35,6 +35,7 @@ jobs:
       - name: Upload artifact
         uses: actions/upload-pages-artifact@v1
   deploy:
+    if: ${{ github.ref == 'refs/heads/main' }}
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}