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>

Documentation/gitcore-tutorial.txt

@@ -1429,7 +1429,7 @@ Although Git is a truly distributed system, it is often
 convenient to organize your project with an informal hierarchy
 of developers. Linux kernel development is run this way. There
 is a nice illustration (page 17, "Merges to Mainline") in
-http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf[Randy Dunlap's presentation].
+https://web.archive.org/web/20120915203609/http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf[Randy Dunlap's presentation].
 
 It should be stressed that this hierarchy is purely *informal*.
 There is nothing fundamental in Git that enforces the "chain of