Fix wrong method call for extend subcommand

Ensure extend is called instead of extend_volume
as the latter doesn't exist.

Change-Id: I5a4ce5904dc73586a124f7bddebc8ffd30841980

cinderclient/v2/shell.py

@@ -915,4 +915,4 @@ def do_transfer_show(cs, args):
 def do_extend(cs, args):
     """Attempt to extend the size of an existing volume."""
     volume = _find_volume(cs, args.volume)
-    cs.volumes.extend_volume(volume, args.new_size)
+    cs.volumes.extend(volume, args.new_size)