exit/kthread: Fix the kerneldoc comment for kthread_complete_and_exit

I misspelled kthread_complete_and_exit in the kernel doc comment fix
it.

Link: https://lkml.kernel.org/r/202112141329.KBkyJ5ql-lkp@intel.com
Link: https://lkml.kernel.org/r/202112141422.Cykr6YUS-lkp@intel.com
Reported-by: kernel test robot <lkp@intel.com>
Fixes: cead1855

 ("exit: Rename complete_and_exit to kthread_complete_and_exit")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>