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

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.
```