Update Zg_tautau40GeV=mu description

There is a typo in the decay descriptor by S. Farry when it was first created, where the descriptor is for a Z -> tau tau -> e nu_e tau decay, even though the title suggests that the taus decay muonically.

This behaviour is a typo as the decay is controlled by pythia, specifically from

Generation().Special.PythiaHiggsType.TypeOfLepton = [ "mu+" ] 

So the decay descriptor in this file doesn't have an effect.

This typo is confusing as it can mislead people to think that this is an electronic decay rather than a muonic one, and will cause people to reverify this decfile if they ever come across it.

Given that this typo does not affect physics results but is confusing, this MR fixes it. As it does not change physics as well, the event type is not required to be changed.

cc @mvesteri