[css-color-5][editorial] Tweak syntax (#12839)

Author: cdoublev

css-color-5/Overview.bs

@@ -218,7 +218,7 @@ Mixing Colors: the ''color-mix()'' Function {#color-mix}
 
 
 <pre class='prod'>
-	<dfn>color-mix()</dfn> = color-mix( [ <<color-interpolation-method>> ,]? [ <<color>> && <<percentage [0,100]>>? ]#)
+	<dfn>color-mix()</dfn> = color-mix( <<color-interpolation-method>>? , [ <<color>> && <<percentage [0,100]>>? ]#)
 </pre>
 
 <wpt>