Fix the help comment about metadata

If passing metadata with "--meta" option, the metadata is stored into
meta_data.json file on the metadata server like:

 $ curl http://169.254.169.254/openstack/latest/meta_data.json
 {
     "hostname": "vm01.novalocal",
     "meta": {"key01": "value01", "key02": "value02"},
     "name": "vm01",
     [..]
 }

This patch fixes the help comment.

Change-Id: I7359d4c2b0aec6d5d07be7938d0000ca23c93ff0
Merge "Fix the help comment about metadata"