Fix html5/block_admonition – icons should have class “fa icon-”

I’ve incorrectly fixed it together with ulist markers in 2c004a3e9ce5b480beaf8bb77dfa0e4c374c9c92, but it actually should be “fa icon-” – according to [this line](https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor/converter/html5.rb#L328) in the built-in and the stylesheet.