Fix typo in upgrade playbook's name.
Fix typo in upgrade playbook's name.

Fix typo to avoid error when wrong playbook is referenced:
  openstack overcloud upgrade run \
    --stack overcloud --roles Controller \
    --playbook post_upgrade_steps_playbooks.yaml
  ...
  u'ERROR! playbooks must be a list of plays'

Change-Id: I2a02d567bbc9e95c198d8cfe3e368d97b43be9a9

openstack/python-tripleoclient