Update groff_tmac(5)

[Peter Schaffter]

I don't look at groff_tmac very often.  I just noticed this entry
for the mom macro package:

  mom  The new mom macro package, only available in groff.  As this
       is not based on other packages, it can be freely designed.
       So it is expected to become quite a nice, modern macro package.
       See groff_mom(7).

Mom's been around for almost two decades.  She's no longer the new
kid on the block.  Isn't it time this entry be updated?

--
Peter Schaffter
https://www.schaffter.ca