Update API example to Bespoke.js v0.4.0

`bespoke.horizontal.from()` has been deprecated in Bespoke.js v0.4.0, but I forgot to update the API example in the readme generated by [Bespoke.js Plugin Generator](https://github.com/markdalgleish/generator-bespokeplugin).

I've updated the generator, but I'm fixing my mistake in your repo too :)