drm/doc: Remove <term> from rendernode docs

The stylesheet doesn't allow this in normal paragraphs.

Cc: David Herrmann <dh.herrmann@gmail.com>
Acked-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>