[typo] Fix threading.Barrier comment that used confusing punctuation (GH-28623) (GH-28625)

Removed extra comma in comment that indicates state of a `Barrier` as it was confusing and breaking the flow while reading.

Co-authored-by: Priyank <5903604+cpriyank@users.noreply.github.com>
(cherry picked from commit f1ca5d7f61594bf3627f0897b596877a0774c8e7)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>