[css-values-5] Group the first arg together in color-mix(). w3c#10867

Author: tabatkins

css-values-5/Overview.bs

@@ -797,7 +797,7 @@ Interpolated Color Values: the ''color-mix()'' notation</h3>
 
 	<pre class=prod>
 		<<color-mix()>> =
-		  color-mix( <<progress>> && <<color-interpolation-method>>?, <<color>>, <<color>> ) |
+		  color-mix( [ <<progress>> && <<color-interpolation-method>>? ] , <<color>>, <<color>> ) |
 		  color-mix( <<color-interpolation-method>>, [<<color>> && <<percentage [0,100]>>?]#{2} )
 	</pre>