Fix syntax in link-checker call

Author: brian-rose

.github/workflows/nightly-build.yaml

@@ -26,4 +26,4 @@ jobs:
     if: ${{ github.repository_owner == 'ProjectPythia' }}
     uses: ProjectPythia/cookbook-actions/.github/workflows/link-checker.yaml@main
     with:
-      path_to_source: 'portal'
+      path_to_notebooks: 'portal'