drm/doc: fix missing verb

Add a missing "be". While there, also fix the syntax for
struct drm_device.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190313153537.22654-2-luca@lucaceresoli.net