[css-values] Serialize % before px in calc example

[ewilligers]

The example ''calc(20px + 0%)'' is serialized as
''calc(0% + 20px)'', according to the serialize a summation rules
https://drafts.csswg.org/css-values/#math-function-serialize-a-summation

i.e. Sort the terms in the following order:

1. The number, if present
2. The percentage, if present
3. The dimensions, ordered by their units <a>ASCII case-insensitive</a> alphabetically
4. The ''min()'' and ''max()'' functions,
	in the order they appeared in the original expression.