[css-color] Linger reference to rgb() with optional alpha, which the WG rejected.

Author: tabatkins

css-color/Overview.bs

@@ -866,7 +866,7 @@ HSL Colors: ''hsl()'' and ''hsla()'' functions</h2>
 	but this specification does not define how to do so.
 
 	The final argument specifies the alpha channel of the color.
-	It's interpreted identically to the fourth argument of the ''rgb()'' function.
+	It's interpreted identically to the fourth argument of the ''rgba()'' function.
 	If omitted, it defaults to ''100%''.
 
 	<div class='example'>
@@ -1314,7 +1314,7 @@ HWB Colors: ''hwb()'' function</h2>
 	with the same relative ratio.
 
 	The fourth argument specifies the alpha channel of the color.
-	It's interpreted identically to the fourth argument of the ''rgb()'' function.
+	It's interpreted identically to the fourth argument of the ''rgba()'' function.
 	If omitted, it defaults to ''100%''.
 
 	The resulting color can be thought of conceptually as a mixture of paint in the chosen hue,