gitcore-tutorial: update broken link

gitcore-tutorial: update broken link

The slides for the Linux-mentoring presentation are no
longer available. Let's point to the wayback version of the
page, which works.

Note that the referenced diagram is also available on page
15 of [1]. We could link to that instead, but it's not clear
from the URL scheme ("uploads") whether it's going to stick
around forever.

[1] https://www.linuxfoundation.jp/jp_uploads/seminar20070313/Randy.pdf

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>