drm/doc: Fix misplaced </para>

Oops. This is a regression from

commit 5d7a9515
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jan 4 22:31:20 2013 +0100

    drm/doc: updates for new framebuffer lifetime rules

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>