doc: replace or.cz gitwiki link with git.wiki.kernel.org

The or.cz version of the Git wiki went away long ago, and
now just redirects to kernel.org.

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

Documentation/git-tools.txt

@@ -7,4 +7,4 @@ maintained here. These days, however, search engines fill that role much
 more efficiently, so this manually-maintained list has been retired.
 
 See also the `contrib/` area, and the Git wiki:
-http://git.or.cz/gitwiki/InterfacesFrontendsAndTools
+https://git.wiki.kernel.org/index.php/InterfacesFrontendsAndTools