creating a TOC entry for mobile app under tutorials

Author: codemillmatt

articles/cosmos-db/TOC.yml

@@ -51,6 +51,8 @@
           href: sql-api-java-application.md
         - name: Node.js
           href: sql-api-nodejs-application.md
+      - name: Build a mobile app
+        items:
         - name: Xamarin
           href: mobile-apps-with-xamarin.md
     - name: 2 - Migrate data