Trivial: Fix wrong param name in comment.

"volume" should be "attachments".

Change-Id: I41c797f0c7cccb2727e9e6cd6424ea2f740624b7

Author: tang-chen

openstackclient/volume/v1/volume.py

@@ -239,7 +239,7 @@ def take_action(self, parsed_args):
         def _format_attach(attachments):
             """Return a formatted string of a volume's attached instances
 
-            :param volume: a volume.attachments field
+            :param attachments: a volume.attachments field
             :rtype: a string of formatted instances
             """