Fixed spelling in a docstring

cmd2/command_definition.py

@@ -98,7 +98,7 @@ def __init__(self) -> None:
     def on_register(self, cmd: 'cmd2.Cmd') -> None:
         """
         Called by cmd2.Cmd as the first step to registering a CommandSet. The commands defined in this class have
-        not be added to the CLI object at this point. Subclasses can override this to perform any initialization
+        not been added to the CLI object at this point. Subclasses can override this to perform any initialization
         requiring access to the Cmd object (e.g. configure commands and their parsers based on CLI state data).
 
         :param cmd: The cmd2 main application