update the example to match prism examples structure

[JaKXz]

As documented here: https://prismjs.com/#basic-usage

and fixes #137 a small typo :)

[JaKXz]

@dekked the getLineProps and getTokenProps methods actually return the key prop and put it on the rendered element, if I understand correctly :)

[dekked]

Oh, that's right... but I still get my linter complaining :)

[JaKXz]

I'm not sure this applies anymore (because of some writing I found about the code and pre elements)... @jpdriver happy to reopen if you think it's useful!