Trivial: Fix wrong param name in comment.

"volume" should be "attachments".

Change-Id: Id1e9a733e18db595d8981b5b3a7735313a346787

Author: tang-chen

openstackclient/volume/v2/volume.py

@@ -245,7 +245,7 @@ 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
             """