Documentation: update stable tree link

Documentation: update stable tree link

BugLink: https://bugs.launchpad.net/bugs/1969110
commit 555d44932c67e617d89bc13c81c7efac5b51fcfa upstream.

The link to stable tree is redirected to
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git. Update
accordingly.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20220314113329.485372-6-bagasdotme@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 72403d1c04bb21c5dbab2210d38aed6ee0453b5d)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>