Fix swerv.config help for ret_stack_size

[tomverbeure]

When running swerv.config -h, you get the following:

Additional direct options for the following variables:

     -ret_size =      {2, 3, 4, ... 8}
        size of return stack

In reality, the tool expects -ret_stack_size instead of -ret_size.

[olofk]

FYI, this is the same fix as the first commit in #2

[aprnath]

Thanks.