Fix the default documented value for the `speed` option

[wardpeet]

pngquant's default speed has changed to 4 instead of 3 https://github.com/kornelski/pngquant/blob/4219956d5e080be7905b5581314d913d20896934/rust/bin.rs#L61

It's an old PR in the gatsby repo so I thought lets clean it up here as well.