Fix --os-auth-plugin in auth_with_unscoped_saml

The error message refers to --os-auth-plugin which is not a valid
option. This patch changes that to --os-auth-type.

Change-Id: I02ec0b7855131180bb8c674051930ebb51cb7303
Closes-Bug: #1477083

Author: hughsaunders

openstackclient/identity/v3/unscoped_saml.py

@@ -37,7 +37,7 @@ def _decorated(self, parsed_args):
         else:
             msg = ('This command requires the use of an unscoped SAML '
                    'authentication plugin. Please use argument '
-                   '--os-auth-plugin with one of the following '
+                   '--os-auth-type with one of the following '
                    'plugins: ' + ', '.join(UNSCOPED_AUTH_PLUGINS))
             raise exceptions.CommandError(msg)
     return _decorated