[css-color-5] Re-introduce commas per WG resolution, closes w3c#6066

Author: LeaVerou

css-color-5/Overview.bs

@@ -121,7 +121,7 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 
 
 <pre class='prod'>
-	<dfn>color-mix()</dfn> = color-mix( in <<colorspace>> && [ [ <<color>> && [ <<percentage>> ]? ] <<color>> ])
+	<dfn>color-mix()</dfn> = color-mix( in <<colorspace>> , [ <<color>> && <<percentage>>? ]#{2})
 </pre>
 
 If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).