Update tutorial-2.md (#6736)

Make clear how we create the test probe in an xunit test

Author: richard-smith-67

docs/articles/intro/getting-started/tutorial-2.md

@@ -180,7 +180,7 @@ Putting read and write protocol together, the device actor will look like this:
 [!code-csharp[Main](../../../../src/core/Akka.Docs.Tutorials/Tutorial2/Device.cs?name=full-device)]
 
 We are also responsible for writing a new test case now, exercising both the read/query and write/record functionality
-together:
+together. Note, that your test class should implement Akka.TestKit.XUnit2.TestKit in order to access CreateTestProbe().
 
 [!code-csharp[Main](../../../../src/core/Akka.Docs.Tutorials/Tutorial2/DeviceSpec.cs?name=device-write-read-test)]