"hljs-source" needs to be changed to "source" in the README

Looking at the [live example](http://pc035860.github.io/angular-highlightjs/example/#/hljs-source), it looks like `hljs-source` does not work and `source` must be used instead. Here is what I ended up with which works:

``` html
<div hljs hljs-language='json' source="'{{queueDataModal.queueData | json}}'"></div>
```

If you'd like me to do this and submit a pull request, let me know.
