Updated master help message for acls.

  --acls=VALUE                             The value could be a JSON-formatted string of ACLs
                                           or a file path containing the JSON-formatted ACLs used
                                           for authorization. Path could be of the form 'file:///path/to/file'
                                           or '/path/to/file'.
                                           
                                           Note that if the flag --authorizers is provided with a value different
                                           than 'local', the ACLs contents will be ignored.
                                           
                                           See the ACLs protobuf in mesos.proto for the expected format.