[css-images-4] Make explicit what the default interpolation color space is. w3c#7948

Author: svgeesus

css-images-4/Overview.bs

@@ -1758,7 +1758,7 @@ Coloring the Gradient Line</h4>
 	using premultiplied alpha, as defined in [[css-color-4#interpolation-alpha]].
 	If no <<color-interpolation-method>> is specified in the gradient function,
 	the color space used for gradient interpolation
-	is the default interpolation color space as defined in [[css-color-4]].
+	is the default interpolation color space, Oklab, as defined in [[css-color-4]].
 
 	By default,
 	this interpolation is linear--