shells/mksh: Fix MASTER_SITES

Replace 1st site with up-to-date one and keep
the 2nd site as a potential backup for now.

PR: 292318
Reported by: linimon@
Authored by: schaiba@gmail.com (maintainer)
Reviewed by: osa, vvd (mentors)
Approved by: vvd (mentor)
Differential Revision: https://reviews.freebsd.org/D55543