Fixed typo in docs/topics/testing/tools.txt

docs/topics/testing/tools.txt

@@ -1332,7 +1332,7 @@ applications:
 The failure messages given by most of these assertion methods can be customized
 with the ``msg_prefix`` argument. This string will be prefixed to any failure
 message generated by the assertion. This allows you to provide additional
-details that may help you to identify the location and cause of an failure in
+details that may help you to identify the location and cause of a failure in
 your test suite.
 
 .. method:: SimpleTestCase.assertRaisesMessage(expected_exception, expected_message, callable, *args, **kwargs)