doc: update list archive reference to use lore.
No disrespect to other mailing list archives, but the local part of
their URLs will become pretty much meaningless once the archives go
out of service, and we learned the lesson hard way when $gmane
stopped serving.
Let's point into https://lore.kernel.org/ for an article that can be
found there, because the local part of the URL has the Message-Id:
that can be used to find the same message in other archives, even if
lore goes down.
Signed-off-by: Junio C Hamano <gitster@pobox.com>