Update quickstart-serverless.md

Author: vicancy

articles/azure-web-pubsub/quickstart-serverless.md

@@ -345,6 +345,7 @@ In this tutorial, you learn how to:
      ```
    - Add `using` statements in header to resolve required dependencies.
      ```c#
+     using System.Threading.Tasks;
      using Microsoft.Azure.WebJobs.Extensions.WebPubSub;
      using Microsoft.Azure.WebPubSub.Common;
      ```
@@ -565,4 +566,4 @@ In this quickstart, you learned how to run a serverless chat application. Now, y
 > [Quick start: Create a simple chatroom with Azure Web PubSub](./tutorial-build-chat.md)
 
 > [!div class="nextstepaction"] 
-> [Explore more Azure Web PubSub samples](https://github.com/Azure/azure-webpubsub/tree/main/samples)
+> [Explore more Azure Web PubSub samples](https://github.com/Azure/azure-webpubsub/tree/main/samples)