Update README.md (#628)

Update README to reflect changes from Moq to NSubstitute

README.md

@@ -247,7 +247,7 @@ Test projects could be organized based on the kind of test (unit, functional, in
 
 - [xunit](https://www.nuget.org/packages/xunit) I'm using xunit because that's what ASP.NET Core uses internally to test the product. It works great and as new versions of ASP.NET Core ship, I'm confident it will continue to work well with it.
 
-- [Moq](https://www.nuget.org/packages/Moq/) I'm using Moq as a mocking framework for white box behavior-based tests. If I have a method that, under certain circumstances, should perform an action that isn't evident from the object's observable state, mocks provide a way to test that. I could also use my own Fake implementation, but that requires a lot more typing and files. Moq is great once you get the hang of it, and assuming you don't have to mock the world (which we don't in this case because of good, modular design).
+- [NSubstitute](https://www.nuget.org/packages/NSubstitute) I'm using NSubstitute as a mocking framework for white box behavior-based tests. If I have a method that, under certain circumstances, should perform an action that isn't evident from the object's observable state, mocks provide a way to test that. I could also use my own Fake implementation, but that requires a lot more typing and files. NSubstitute is great once you get the hang of it, and assuming you don't have to mock the world (which we don't in this case because of good, modular design).
 
 - [Microsoft.AspNetCore.TestHost](https://www.nuget.org/packages/Microsoft.AspNetCore.TestHost) I'm using TestHost to test my web project using its full stack, not just unit testing action methods. Using TestHost, you make actual HttpClient requests without going over the wire (so no firewall or port configuration issues). Tests run in memory and are very fast, and requests exercise the full MVC stack, including routing, model binding, model validation, filters, etc.