media: mc: entity: Fix doc for media_graph_walk_init

There is no media_graph_walk_free(). media_graph_walk_cleanup() is used
to release the resources.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>