Clarify --cluster-name parameter documentation for export_db.py

[snowch]

Instead of:

--cluster-name CLUSTER_NAME
                        Cluster name to export the metastore to a specific
                        cluster. Cluster will be started.

this would be clearer:

-cluster-name CLUSTER_NAME
                        Cluster name to use for exporting the metastore.
                        Cluster will be started.