Fixed path in Devstack plugin README
Fixed path in Devstack plugin README

README suggested that stack.sh script is in /opt/stack/magnum
instead of /opt/stack/devstack.

Change-Id: I030189a485c401d44dc8c5ad637b216ed91e71eb

ramielrowe/magnum,dimtruck/magnum,openstack/magnum,annegentle/magnum,ramielrowe/magnum,jay-lau/magnum,ChengTiesheng/magnum,ArchiFleKs/magnum,eshijia/magnum,sajuptpm/magnum,Tennyson53/SUR,annegentle/magnum,Tennyson53/SUR,paulczar/magnum,hongbin/magnum,mjbrewer/testindex,eshijia/magnum,mjbrewer/testindex,ArchiFleKs/magnum,openstack/magnum,ffantast/magnum,Tennyson53/magnum,Alzon/SUR,LaynePeng/magnum,Alzon/SUR,paulczar/magnum,Tennyson53/magnum,ddepaoli3/magnum,mjbrewer/testindex,eshijia/SUR,ChengTiesheng/magnum,ffantast/magnum,dimtruck/magnum,eshijia/SUR,hongbin/magnum,LaynePeng/magnum,ddepaoli3/magnum,sajuptpm/magnum