CrimeRateUpdate class uses 'overlay' list in update function

[ldicker83]

This is a bit ambiguous -- while the behavior is correct calling it an overlay list is a misnomer.

This could be done better by renaming the parameter to something making it clear that it's a list of tiles with police coverage or by providing a list of police structures and generating the tile list internally.

Primarily this is a documentation issue focused on improving possible ambiguity.