Fix typo in pluginguide
Fix typo in pluginguide

Change-Id: I84c4b1db388ce2ccfab9dabbba6f90aea60a4c22

jasondunsmore/heat,cryptickp/heat,rh-s/heat,steveb/heat,noironetworks/heat,maestro-hybrid-cloud/heat,dims/heat,openstack/heat,cwolferh/heat-scratch,pshchelo/heat,steveb/heat,cryptickp/heat,dragorosson/heat,srznew/heat,rdo-management/heat,takeshineshiro/heat,pratikmallya/heat,gonzolino/heat,miguelgrinberg/heat,jasondunsmore/heat,pshchelo/heat,gonzolino/heat,miguelgrinberg/heat,dims/heat,openstack/heat,dragorosson/heat,rh-s/heat,srznew/heat,noironetworks/heat,rdo-management/heat,maestro-hybrid-cloud/heat,cwolferh/heat-scratch,takeshineshiro/heat,pratikmallya/heat