Update testing.md for a grammar issue. (#2573)

* Update testing.md for a grammar issue.

* Update testing.md

aspnetcore/mvc/controllers/testing.md

@@ -44,7 +44,7 @@ Typical controller responsibilities:
 
 [Unit testing](https://docs.microsoft.com/en-us/dotnet/articles/core/testing/unit-testing-with-dotnet-test) involves testing a part of an app in isolation from its infrastructure and dependencies. When unit testing controller logic, only the contents of a single action is tested, not the behavior of its dependencies or of the framework itself. As you unit test your controller actions, make sure you focus only on its behavior. A controller unit test avoids things like [filters](filters.md), [routing](../../fundamentals/routing.md), or [model binding](../models/model-binding.md). By focusing on testing just one thing, unit tests are generally simple to write and quick to run. A well-written set of unit tests can be run frequently without much overhead. However, unit tests do not detect issues in the interaction between components, which is the purpose of [integration testing](xref:mvc/controllers/testing#integration-testing).
 
-If you've writing custom filters, routes, etc, you should unit test them, but not as part of your tests on a particular controller action. They should be tested in isolation.
+If you're writing custom filters, routes, etc, you should unit test them, but not as part of your tests on a particular controller action. They should be tested in isolation.
 
 > [!TIP]
 > [Create and run unit tests with Visual Studio](https://www.visualstudio.com/en-us/get-started/code/create-and-run-unit-tests-vs).