Fix cluster import error message

[jpetitcolas]

When importing a cluster through its ID:

terraform import qovery_cluster.cluster <cluster_id>

It currently raises an error:

Expected import identifier with format: cluster_id,organization_id. Got: "efb1a047-773f-428d-8d79-a4295d64617d"

However, the expected format seems to be organization ID/cluster ID (like on this command for instance).