Update instructions for get calendar data part of tutorial

Author: rob-windsor

Demos/03-add-msgraph/graph-tutorial/Controllers/CalendarController.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using graph_tutorial.Helpers;
 using System.Threading.Tasks;
 using System.Web.Mvc;

tutorial/add-ms-graph.md

@@ -77,6 +77,7 @@ Consider what this code is doing.
 Now create a controller for the calendar views. Right-click the **Controllers** folder in Solution Explorer and choose **Add > Controller...**. Choose **MVC 5 Controller - Empty** and choose **Add**. Name the controller `CalendarController` and choose **Add**. Replace the entire contents of the new file with the following code.
 
 ```cs
+using System;
 using graph_tutorial.Helpers;
 using System.Threading.Tasks;
 using System.Web.Mvc;