[user-guide] Update the deprecated parameter
[user-guide] Update the deprecated parameter

"--key_name" is deprecated. It should be "--key-name".

Change-Id: I51e8fa3e42c48da6ac98700cf3e492aa9e328e5b

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals