Updated source os-subnets.wadl to correct code sample errors

Fixed these issues in source WADL:

Update subnet's content is for "create subnet"
create subnet's contents is for list subnet
List subnet's contents is for show subnet

Closes-Bug: #1292359

Change-Id: Icf38c79bf32877d0f3f3cb119d31fea2e0b1d61e
author: diane fleming