Update README.md (#84)

fixed LICENSE file extension in Link

Author: vyaspranjal33

README.md

@@ -2,7 +2,7 @@
 
 [![Travis Badge](https://travis-ci.org/sendgrid/csharp-http-client.svg?branch=master)](https://travis-ci.org/sendgrid/csharp-http-client)
 [![NuGet](https://img.shields.io/nuget/v/SendGrid.CSharp.Http.Client.svg)](https://www.nuget.org/packages/SendGrid.CSharp.HTTP.Client)
-[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE.txt)
+[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE.md)
 [![Twitter Follow](https://img.shields.io/twitter/follow/sendgrid.svg?style=social&label=Follow)](https://twitter.com/sendgrid)
 [![GitHub contributors](https://img.shields.io/github/contributors/sendgrid/csharp-http-client.svg)](https://github.com/sendgrid/csharp-http-client/graphs/contributors)