update testing.md - missing import

import for RequestsClient is missing in the docs

docs/api-guide/testing.md

@@ -201,6 +201,7 @@ live environment. (See "Live tests" below.)
 This exposes exactly the same interface as if you were using a requests session
 directly.
 
+    from rest_framework.test import RequestsClient
     client = RequestsClient()
     response = client.get('http://testserver/users/')
     assert response.status_code == 200