vhost-iotlb: fix vhost_iotlb_itree_next() documentation

This patch contains trivial changes for the vhost_iotlb_itree_next()
documentation, fixing the function name and the description of
first argument (@map).

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20200825130543.43308-1-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>