doc: don't use git.kernel.org as example gitweb URL

git.kernel.org uses cgit, not gitweb, these days:

    $ w3m -dump 'http://git.kernel.org/?p=git/git.git;a=tree;f=gitweb' | grep -w generated
    generated by cgit 1.2-0.3.lf.el7 (git 2.18.0) at 2019-06-22 16:14:38 +0000

Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitweb.txt

@@ -28,8 +28,7 @@ Gitweb provides a web interface to Git repositories.  Its features include:
   revisions one at a time, viewing the history of the repository.
 * Finding commits which commit messages matches given search term.
 
-See http://git.kernel.org/?p=git/git.git;a=tree;f=gitweb[] or
-http://repo.or.cz/w/git.git/tree/HEAD:/gitweb/[] for gitweb source code,
+See http://repo.or.cz/w/git.git/tree/HEAD:/gitweb/[] for gitweb source code,
 browsed using gitweb itself.