Update toc.yml

Author: jaesoni

articles/application-gateway/toc.yml

@@ -289,6 +289,8 @@
       href: ../reliability/migrate-app-gateway-v2.md?toc=/azure/application-gateway/toc.json
   - name: Proxy buffer configuration
     href: proxy-buffers.md
+  - name: Using Server-sent events
+    href: using-sse.md    
   - name: Custom error pages
     href: custom-error.md
   - name: Migrate to v2 SKU