vringh: fix typos in the vringh_init_* documentation

Replace `userpace` with `userspace`.

Cc: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230331080208.17002-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>