Fix legacy naming: dotfiles-add -> mkrc
Fix legacy naming: dotfiles-add -> mkrc

The very original name of the `mkrc` script was `dotfiles-add`, and that
name was still in the help text. Fixed.

Pointed out by George Brocklehurst.

fgandellini/rcm,ali/rcm,fgandellini/rcm,zachlatta/rcm,ali/rcm,Frost/rcm,ali/rcm,tejasmanohar/rcm,TheLinuxKitten/rcm,tejasmanohar/rcm,devm33/rcm,zachlatta/rcm,devm33/rcm,TheLinuxKitten/rcm,tejasmanohar/rcm,thoughtbot/rcm,Frost/rcm,fgandellini/rcm,thoughtbot/rcm,fgandellini/rcm,zachlatta/rcm,zachlatta/rcm,tejasmanohar/rcm,devm33/rcm,Frost/rcm,thoughtbot/rcm,TheLinuxKitten/rcm,ali/rcm,TheLinuxKitten/rcm,devm33/rcm,Frost/rcm