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

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.

Author: ericwilligers

css-values-4/Overview.bs

@@ -2442,7 +2442,7 @@ Serialization</h4>
 		For example, ''calc(20px + 30px)'' would serialize as ''calc(50px)'' as a specified value,
 		or as ''50px'' as a computed value.
 
-		A value like ''calc(20px + 0%)'' would serialize as ''calc(20px + 0%)'',
+		A value like ''calc(20px + 0%)'' would serialize as ''calc(0% + 20px)'',
 		maintaining both terms in the serialized value.
 		(It's important to maintain zero-valued terms,
 		so the ''calc()'' doesn't suddenly "change shape" in the middle of a transition