Changed stdout documentation to detail the 's' paramater

Changed `stdout` documentation to reference the `s` parameter. `stdout` will no longer be flagged by rdoc as having an undocumented parameter.

This is my first Ruby documentation commit so let me know if I need to change anything :)
@vipulnsward Yes I think that does look better. I was thinking about that but didn't know whether it was called `s` for a reason. I've updated my PR to reflect the change that you suggested.