drm/writeback: Fix the "overview" section of the doc

Fix the bullet list declaration in the overview section.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 935774cd ("drm: Add writeback connector type")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703174046.22396-1-boris.brezillon@bootlin.com