fix a typo from on_trait_change to observe migration

[aaronayres35]

In #1519 there was a typo that went unnoticed. The tests all still passed but in the test suite one could see the error

Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<traitsui.key_bindings.KeyBindings object at 0x7f9344a73048>, name='children', old=[<traitsui.key_bindings.KeyBindings object at 0x7f935c6c6ca8>], new=[])
Traceback (most recent call last):
  File "/Users/aayres/Desktop/traits/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/aayres/Desktop/traits/traits/observation/observe.py", line 26, in dispatch_same
    handler(event)
  File "/Users/aayres/Desktop/traitsui/traitsui/key_bindings.py", line 267, in _children_modified
    for item in event.added:
AttributeError: 'TraitChangeEvent' object has no attribute 'added'
.Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<traitsui.key_bindings.KeyBindings object at 0x7f9344ac13b8>, name='children', old=[<traitsui.key_bindings.KeyBindings object at 0x7f934532edb0>], new=[])
Traceback (most recent call last):
  File "/Users/aayres/Desktop/traits/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/aayres/Desktop/traits/traits/observation/observe.py", line 26, in dispatch_same
    handler(event)
  File "/Users/aayres/Desktop/traitsui/traitsui/key_bindings.py", line 267, in _children_modified
    for item in event.added:
AttributeError: 'TraitChangeEvent' object has no attribute 'added'

This was because the object is actually a KeyBindings not a KeyBinding so we ended up in the wrong conditional path.

[rahulporuri]

Edit : The code change makes sense to me but I don't see any traceback when I run the testsuite on the main branch either. Not sure what's going on.

LGTM. I ran the testsuite locally and I dont see the traceback!

[aaronayres35]

Edit : The code change makes sense to me but I don't see any traceback when I run the testsuite on the main branch either. Not sure what's going on.

See lines 597-615 in
log (4).txt (pulled from CI run on #1546
For an example of it occurring on CI