Updated CHANGELOG.

Author: kmvanbrunt

CHANGELOG.md

@@ -72,6 +72,7 @@ prompt is displayed.
     - Renamed `cmd2_handler` to `cmd2_subcmd_handler` in the `argparse.Namespace` for clarity.
     - Removed `Cmd2AttributeWrapper` class. `argparse.Namespace` objects passed to command functions
       now contain direct attributes for `cmd2_statement` and `cmd2_subcmd_handler`.
+    - Renamed `cmd2/command_definition.py` to `cmd2/command_set.py`.
     - Removed `Cmd.doc_header` and the `with_default_category` decorator. Help categorization is now
       driven by the `DEFAULT_CATEGORY` class variable (see **Simplified command categorization** in
       the Enhancements section below for details).