[css-lists-3] Use comma-separated values in dfn-for (w3c#7081)

css-lists-3/Overview.bs

@@ -872,7 +872,7 @@ Manipulating Counter Values: the 'counter-increment' and 'counter-set' propertie
 	if there is no counter of the given name on the element yet.
 	Their values are defined as follows:
 
-	<dl dfn-type=value dfn-for="counter-set counter-increment">
+	<dl dfn-type=value dfn-for="counter-set,counter-increment">
 		<dt><dfn>none</dfn>
 		<dd>
 			This element does not alter the value of any counters.