drm: fix trivial field description cut-and-paste error

As reported by Amarnath Baliyase, the drm_mode_status enumeration
documentation describes MODE_V_ILLEGAL as "mode has illegal horizontal
timings".  But that's just a cut-and-paste error from the previous line.
The "V" stands for vertical, of course.

I'm just fixing this directly rather than bothering with going through
the proper channels.  Less work for everybody.

Reported-by: Amarnath Baliyase <baliyaseamarnath@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>