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>

README.md

@@ -33,7 +33,7 @@ requests, comments and patches to git@vger.kernel.org (read
 [Documentation/SubmittingPatches][] for instructions on patch submission).
 To subscribe to the list, send an email with just "subscribe git" in
 the body to majordomo@vger.kernel.org. The mailing list archives are
-available at http://news.gmane.org/gmane.comp.version-control.git/,
+available at https://public-inbox.org/git,
 http://marc.info/?l=git and other archival sites.
 The maintainer frequently sends the "What's cooking" reports that