[css-grid] Percentage line in propdef table for grid-template-{columns,rows}

Hi,

The property definition table for grid-template-columns and 
grid-template-rows says "Percentages: n/a", even though the value can 
contain <percentage>.

I guess "Percentages: See prose" should work. The definition is complex 
enough that it probably shouldn’t be duplicated.

-- 
Simon Sapin