Update docstring

Author: wasade

labman/db/plate.py

@@ -198,7 +198,8 @@ def list_plates(plate_types=None, only_quantified=False,
         -------
         list of dicts
             The list of plate information with the structure:
-            [{'plate_id': int, 'external_id': string}]
+            [{'plate_id': int, 'external_id': string,
+              'creation_timestamp': datetime}]
         """
         with sql_connection.TRN as TRN:
             sql_where, sql_discard, sql_plate_types = '', '', ''