[2.1.x] Fixed typo in patch_logger() docstring.

Backport of 6275b50ac2 from master

django/test/utils.py

@@ -640,7 +640,7 @@ def patch_logger(logger_name, log_level, log_kwargs=False):
     Context manager that takes a named logger and the logging level
     and provides a simple mock-like list of messages received.
 
-    Use unitttest.assertLogs() if you only need Python 3 support. This
+    Use unittest.assertLogs() if you only need Python 3 support. This
     private API will be removed after Python 2 EOL in 2020 (#27753).
     """
     calls = []