Variable Inspector example used a wrong callback argument signature

[NichtJens]

I ran into an issue described here ipython/ipython#12214 for the Variable Inspector example.

The inspector.close() line would crash with the slightly cryptic message:

ValueError: Function <bound method VariableInspectorWindow._fill of <__main__.VariableInspectorWindow object at 0x2b7148c38a58>> is not registered as a post_run_cell callback

... despite the callback clearly being registered.

The fix is to a accept a second argument in the callback.

[jasongrout]

Closed/reopened to trigger another test run.