Fix "neutron subnet-create" usages

"neutron subnet-create" command has some options which names contain
alphabet chars and hyphens, but the samples on the admin guide contain
underscores.
This patch fixes these typos.

Change-Id: I8cc780934f3cf5c48c17112cd7d4cd5aa82e478b
Merge "Fix "neutron subnet-create" usages"