docs/css-overview: Fix color functions’ arg orders

docs/css-overview: Fix color functions’ arg orders

shade/alpha/mix() take colour(s) and a number that is the ratio by which
to transform them. It was written here that these shall be passed in the
order (number, colour). That was wrong: they must be passed in the order
(colour[s], number) to work, and for the Inspector not to flag an error.