Update toc.yml

Author: jaesoni

articles/application-gateway/toc.yml

@@ -261,6 +261,8 @@
           href: add-http-header-rewrite-rule-powershell.md
         - name: Create and rewrite HTTP headers
           href: tutorial-http-header-rewrite-powershell.md
+        - name: Add secure flag for cookies 
+          href: application-gateway-secureflag-sessionaffinity.md       
     - name: URL rewrite
       items:
         - name: Azure portal