Fixes typo in openstack command

openstack user-list changed
to proper form of openstack command

Change-Id: If409ab26bf6266e0790a2d2cce18d28adb844702
Closes-Bug: #1503711

doc/devenv-vagrant.rst

@@ -117,7 +117,7 @@ Deploy multinode with::
 Validate OpenStack is operational::
 
     source ~/openrc
-    openstack user-list
+    openstack user list
 
 Or navigate to http://10.10.10.254/ with a web browser.