Fix typo

StackOverflow -> Stack Overflow

Author: changeworld

articles/azure-web-pubsub/toc.yml

@@ -320,5 +320,5 @@
       href: resource-faq.md
     - name: Microsoft Q&A
       href: /answers/topics/azure-web-pubsub.html
-    - name: StackOverflow
+    - name: Stack Overflow
       href: https://stackoverflow.com/questions/tagged/azure-web-pubsub