ci: fix typo in workflow

Author: machow

.github/workflows/ci.yml

@@ -183,7 +183,7 @@ jobs:
     name: "Preview Docs:"
     runs-on: ubuntu-latest
     needs: ["build-docs"]
-    if: "{{github.event_name == 'pull_request' && !github.event.pull_request.head.repo.fork }}"
+    if: "${{github.event_name == 'pull_request' && !github.event.pull_request.head.repo.fork }}"
 
     steps:
       - uses: actions/download-artifact@v3