sh.1: document optional arguments to :

TIL the : command accepts and discards arguments, and this is
occasionally used to embed comments in interesting places. While
here, fix another ... argument that was not marked up.

MFC after: 3 days
Thanks: Community Discord
Approved by: carlavilla, mhorne (mentors)
Differential Revision: https://reviews.freebsd.org/D48398

(cherry picked from commit f4727eca405763dd1feb4ca3e17be40c2da337de)