[css-color-5] Change color-mix() grammar so that color space cannot be in the middle and that percentage can

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>>{2} && [ <<percentage>> ]?)
+	<dfn>color-mix()</dfn> = color-mix( in <<colorspace>> && [ <<color>> && <<color>> && [ <<percentage>> ]? ])
 </pre>
 
 If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).