ci: update workflow

Author: cybex-dev

.github/workflows/flutter.yml

@@ -59,7 +59,7 @@ jobs:
       - run: cd ./example; flutter build web --release --target=lib/main.dart --output=build/web
 
       - name: Update service worker
-        run: cat ./example/service-worker/twilio-sw.js > ./example/build/web/flutter_service_worker.js
+        run: cat ./example/service-worker/twilio-sw.js >> ./example/build/web/flutter_service_worker.js
 
       - name: Archive Production Artifact
         uses: actions/upload-artifact@master