doc: replace LKML link with lore.kernel.org

doc: replace LKML link with lore.kernel.org

Since we're now recommending lore.kernel.org, replace LKML link
with lore.kernel.org.

Although LKML has been around for a long time, nothing lasts forever
(see Gmane). Since LKML uses opaque message identifiers, switching to
lore.kernel.org should be a strict improvement since, even if
lore.kernel.org goes down, the Message-ID will allow future readers to
look up the referenced messages on any other archive.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>