[css-grid] Clarification about <auto-track-list> and <line-names> syntaxes #2676

Spec: https://drafts.csswg.org/css-grid/#typedef-auto-track-list

The spec defines <auto-track-list> syntax as following:

[ <line-names>? [ <fixed-size> | <fixed-repeat> ] ]* <line-names>? <auto-repeat>
[ <line-names>? [ <fixed-size> | <fixed-repeat> ] ]* <line-names>?

I suppose | combinator is missed in that syntax.

<line-names> is defined as following:

'[' <custom-ident>* ']'

According to this syntax, an empty brackets is a valid value for the <line-names>. It looks odd, I guess * multiplier should be replaced for +.