Use markdown::mark() instead of mark_html(options = '-standalone')

[yihui]

mark() defaults to generating a fragment, so this way is slightly cleaner.

This is quite a trivial change, so probably not worth a mention in NEWS.

PR task list:

• [ ] Update NEWS
• [ ] Add tests (if possible)
• [ ] Update documentation with devtools::document()

[gadenbuie]

Thanks!