replace metavar "volume-id" with "volume" to avoid ambiguity

The help message of command "openstack volume show" accepts either
volume-name or volume-ID. But the metavar is "volume-id" as below.
It can easily lead to misunderstanding.

usage: openstack volume show [-h] [-f {json,shell,table,value,yaml}]
                             [-c COLUMN] [--max-width <integer>]
                             [--noindent][--prefix PREFIX]
                             <volume-id>

Change-Id: I57576ea23868b1026cf268be69b39e98a53aafd4

Author: Shu Yingya

openstackclient/volume/v2/volume.py

@@ -490,7 +490,7 @@ def get_parser(self, prog_name):
         parser = super(ShowVolume, self).get_parser(prog_name)
         parser.add_argument(
             'volume',
-            metavar="<volume-id>",
+            metavar="<volume>",
             help=_("Volume to display (name or ID)")
         )
         return parser