Fix "keystone user-list" using tenant-id parameter

"keystone user-list" command has some options which names contain
alphabet chars and hyphens, but the sample on the training guide
contain underscore.
This patch fixes this typo.

Change-Id: I1666f3e0ee46ebbd2028c93f4681809178a13795
Merge "Fix "keystone user-list" using tenant-id parameter"