Fix tripleo-undercloud plugin custom repo documentation
Fix tripleo-undercloud plugin custom repo documentation

The previous example showcased in the documentation is incorrect, and
will cause the custom repo generation task to fail due to wrong
invocation of yum_repository Ansible module.

Fixing the documentation with the correct parameters needed for the
module.

Change-Id: I28160f8b16119a58826517bb1a48ab1b9f3445d8

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared