Fix typo in testing.md (#6257)

Author: tanoabeleyra

docs/api-guide/testing.md

@@ -119,7 +119,7 @@ Extends [Django's existing `Client` class][client].
 
 ## Making requests
 
-The `APIClient` class supports the same request interface as Django's standard `Client` class.  This means the that standard `.get()`, `.post()`, `.put()`, `.patch()`, `.delete()`, `.head()` and `.options()` methods are all available.  For example:
+The `APIClient` class supports the same request interface as Django's standard `Client` class.  This means that the standard `.get()`, `.post()`, `.put()`, `.patch()`, `.delete()`, `.head()` and `.options()` methods are all available.  For example:
 
     from rest_framework.test import APIClient