Fix help message for "server group delete" command

"Server group delete" command supported deleting multi server
groups, but the help message was "Delete an existing server
group", so I change it in this patch.

Change-Id: I05b1a7e3f29b84b9190e7cc05c01734e3daa7a6d

Author: Huanxuan Ao

doc/source/command-objects/server-group.rst

@@ -31,7 +31,7 @@ Create a new server group
 server group delete
 -------------------
 
-Delete an existing server group
+Delete existing server group(s)
 
 .. program:: server group delete
 .. code-block:: bash

openstackclient/compute/v2/server_group.py

@@ -71,7 +71,7 @@ def take_action(self, parsed_args):
 
 
 class DeleteServerGroup(command.Command):
-    """Delete an existing server group."""
+    """Delete existing server group(s)."""
 
     def get_parser(self, prog_name):
         parser = super(DeleteServerGroup, self).get_parser(prog_name)