kthread: fix documentation build warning

The kerneldoc comment for kthread_create() had an incorrect argument
name, leading to a warning in the docs build.

Correct it, and make one more small step toward a warning-free build.

Link: http://lkml.kernel.org/r/20170724135916.7f486c6f@lwn.netSigned-off-by: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kthread.h

@@ -15,7 +15,7 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
  * @threadfn: the function to run in the thread
  * @data: data pointer for @threadfn()
  * @namefmt: printf-style format string for the thread name
- * @...: arguments for @namefmt.
+ * @arg...: arguments for @namefmt.
  *
  * This macro will create a kthread on the current node, leaving it in
  * the stopped state.  This is just a helper for kthread_create_on_node();