Update hugo.md (#1013)
Update hugo.md (#1013)

The `$css.Data.Integrity` variable exists only, if `fingerprint` was run on the CSS. So wrapping it in a `hugo.IsProduction` check is required for the attribute.
FullHuman/purgecss,FullHuman/purgecss,FullHuman/purgecss,FullHuman/purgecss