Use Substitution Decorator for CustomBusinessMonthEnd

[WillAyd]

This is a follow up to https://github.com/pandas-dev/pandas/pull/21093/files#r188805397 which wasn't working with Py27. Now that that is a thing of the past we should be able to use the more idiomatic Substitution approach to generating this docstring

[Batalex]

Hi,
I would like to give it a try as a first contribution.
It does not seem to be very complex when looking at the contributing guide