doc: recommend lore.kernel.org over public-inbox.org

doc: recommend lore. over public-inbox.org

Since lore.kernel.org now has the same archive as public-inbox.org and
may have more longevity going forward[1], let's recommend people use it
for finding or referencing messages.

[1] https://public-inbox.org/git/20191120195556.GA25189@dcvr/
      or if you like:
    https://lore.kernel.org/git/20191120195556.GA25189@dcvr/

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>