ci(check-whitespace): update stale file top comments

ci(check-whitespace): update stale file top comments

Earlier a066a90d (ci(check-whitespace): restrict to the intended
commits, 2021-07-14) changed the check-whitespace task to stop using a
shallow clone, and cc003621 (ci(check-whitespace): stop requiring a
read/write token, 2021-07-14) changed the way how the errors the task
discovered is signaled back to the user.

They however forgot to update the comment that outlines what is done in
the task. Correct them.

Signed-off-by: Hans Krentel (hakre) <hanskrentel@yahoo.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>