ktest.pl: fix english

Ajdust spelling to more common "mandatory".  Variant "mandidory" is
certainly wrong.

Link: http://lkml.kernel.org/r/20161011073003.GA19476@amd


Signed-off-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>