Fix highlight.js example in readme

Current highlight.js example not work since highlightAuto doesn't take a lang parameter: 

https://github.com/isagalaev/highlight.js/blob/58fc91edbb83708375bda87c02d0fa09b46f4676/src/highlight.js#L427

Would try to use `hljs.highlight(lang, code)` style to pass in the lang but the marked detected language doesn't nearly have a 1-1 mapping with highlightjs. For example, highlight.js doesn't know what `html` or `js` is, will spit out a cryptic `TypeError: Cannot read property 'compiled' of undefined` error if it doesn't know what language you are talking about.