update toc title

Author: vhorne

articles/firewall/toc.yml

@@ -228,7 +228,7 @@ items:
   items:
   - name: FAQ
     href: firewall-faq.yml
-  - name: Known issues
+  - name: Known issues and limitations
     href: firewall-known-issues.md
   - name: Author templates
     href: ../azure-resource-manager/templates/syntax.md