Fix traitlet name in docstring

Author: fcollonval

traitlets/config/application.py

@@ -327,7 +327,7 @@ def _log_default(self):
 
             .. code-block:: python
 
-               c.Application.logging_configuration = {
+               c.Application.logging_config = {
                    'handlers': {
                        'file': {
                            'class': 'logging.FileHandler',