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 :)
`stdout` is more readable. wdyt about changing this to 
```ruby
# Sets the IO used as +stdout+. Defaults to STDOUT
      def stdout=(stdout)
        @stdout = stdout
      end
```