vringh: fix a typo in comments for vringh_kiov

Probably it is a simple copy error from struct vring_iov.

Signed-off-by: Shunsuke Mie <mie@igel.co.jp>
Message-Id: <20230202104248.2040652-1-mie@igel.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>