Update functions-dotnet-dependency-injection.md

While going through code snippet, I found a minor issue, which can cause issue to other who copy code from here.

Author: ivj

articles/azure-functions/functions-dotnet-dependency-injection.md

@@ -80,6 +80,7 @@ using Microsoft.Azure.WebJobs;
 using Microsoft.Azure.WebJobs.Extensions.Http;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Logging;
+using System.Net.Http;
 
 namespace MyNamespace
 {