Improve docs regarding Req

[a8t]

There was a recent change to replace the HTTPotion example with Req, but one reference to HTTPotion was accidentally left in.

I changed that reference, but I also clarified the docs a tiny bit.

Thanks for the library!

[a8t]

actually, this still isn't good. Req uses status, not status_code. I will update again

[bernardd]

Nice one, thanks very much @a8t!