drm/atomic-docs: Spelling fixups

Eric nicely pointed these out, but I failed at git add and lost them.
This fixes up

commit 2f196b7c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 2 16:21:44 2016 +0200

    drm/atomic: Add drm_atomic_crtc_state_for_each_plane_state

to actually do what it says on the tin^Wcommit message.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>