doc: Fix openstack CLI command
doc: Fix openstack CLI command

Replace 'openstack placement aggregate set' with
'openstack resource provider aggregate set'.

Change-Id: If085efd0cfb417764e24e9c7eeb23a4d34bf250f
Closes-Bug: #1821916

klmitch/nova,rahulunair/nova,mahak/nova,mahak/nova,openstack/nova,klmitch/nova,openstack/nova,klmitch/nova,mahak/nova,klmitch/nova,rahulunair/nova,openstack/nova,rahulunair/nova