docs: correct supported OSes for `pedalboard.io.Audiostream` (#371)

I think https://github.com/spotify/pedalboard/commit/0ba5f9e278c21a67896b38fcb4b7227c6d6c7912 missed this string 🙂 

(Almost skipped over this project as well, until the commit history showed that Linux supported was merged a mere two days ago!) 

Also took the liberty of downcasing "pedalboard" and putting it in a monospace font, since this is what the rest of the README does.