Update the documentation and help message for retrieving PURE_CLUSTER_PREFIX in the vsphere-xcopy-volume-populator.

The command was updated from:

printf "px_%.8s" $(oc get storagecluster -A -o=jsonpath='{.items[?(@.spec.cloudStorage.provider=="pure")].status.clusterUid}')

To a simpler version:

printf "px_%.8s" $(oc get storagecluster -A -o=jsonpath='{.items[0].status.clusterUid}'| head -c 8)

Files updated:
- cmd/vsphere-xcopy-volume-populator/README.md
- cmd/vsphere-xcopy-volume-populator/internal/pure/flashArray.go

GitHub PR: https://github.com/kubev2v/forklift/pull/3941