Fix incorrect import

Author: danyeaw

winforms/src/toga_winforms/command.py

@@ -3,8 +3,8 @@
 from System.ComponentModel import InvalidEnumArgumentException
 
 from toga import Command as StandardCommand, Group, Key
+from toga.handlers import WeakrefCallable
 from toga_winforms.keys import toga_to_winforms_key, toga_to_winforms_shortcut
-from toga_winforms.libs.wrapper import WeakrefCallable
 
 
 class Command: