[img] changed the spelling of third party in the image
[img] changed the spelling of third party in the image

In Rally-UseCases.png image, changed the spelling of
"third part" to "third party".

Change-Id: Iab909299fc40b36a9865743622e37aaaefdd0d0d

openstack/rally,openstack/rally,openstack/rally,yeming233/rally,openstack/rally,yeming233/rally