Correct bug in .github docs.yaml

RTD sample link was not posted in updated PR.
Add missing curly brackets to line.

Refs #619

.github/workflows/docs.yaml

@@ -57,7 +57,7 @@ jobs:
         if-no-files-found: ignore
 
     - name: Comment ReadDocs Link in PR
-      if: github.event_name == 'pull_request'
+      if: ${{ github.event_name == 'pull_request' }}
       uses: actions/github-script@v6
       with:
         script: |