README: replace gmane link with public-inbox

README: replace gmane link with public-inbox

The general status and future of gmane is unclear at this
point, but certainly it does not seem to be carrying
gmane.comp.version-control.git at all anymore. Let's point
to public-inbox.org, which seems to be the favored archive
on the list these days (and which uses message-ids in its
URLs, making the links somewhat future-proof).

Reported-by: Chiel ten Brinke <ctenbrinke@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>