Fix broken argument name in v2 volume extend routine

The new_size attribute was misspelled, causing the volume extend
operation to fail with:

    ERROR: 'Namespace' object has no attribute 'new_size'

This happens only with the v2 API, v1 works fine.

Change-Id: Icb6b5008dad229f21e7906d9b5342d41f5e760cd
Closes-Bug: #1259552

cinderclient/v2/shell.py

@@ -1029,7 +1029,7 @@ def do_transfer_show(cs, args):
 
 @utils.arg('volume', metavar='<volume>',
            help='Name or ID of the volume to extend.')
-@utils.arg('new-size',
+@utils.arg('new_size',
            metavar='<new_size>',
            type=int,
            help='New size of volume in GB')