[css-grid] Clarify "CCW" acronym with <abbr>

[Loirooriol]

https://drafts.csswg.org/css-grid/#propdef-grid-area has a note:

The resolution order [...] goes CCW for LTR pages

I guess CCW means "CounterClockWise" but someone might understand "Carrying a Concealed Weapon" or various other things.

If you want to use an acronym please describe it with <abbr> for clarity.