Documentation: kunit: change complete_and_exit to kthread_complete_and_exit

Commit cead1855 ("exit: Rename complete_and_exit to
kthread_complete_and_exit") renamed complete_and_exit to
kthread_complete_and_exit.

Signed-off-by: Andreas-Christian Hagau <ach@hagau.se>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>