drm/doc: Small markup fixup

Drive-by cleanup.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170404095304.17599-11-daniel.vetter@ffwll.ch
1 file changed