help: print long options with a single dash as well

we simply strip the second dash, so make the doc consistent with the
other tools.

Task-number: QTBUG-24626
Change-Id: I3286808094dc9c64ade821a3db797cb207e0eac9
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Gabor Garami