[css-images-4][editorial] Use radial- rather than rg-

Follow-up on 69197fb.

Author: cdoublev

css-images-4/Overview.bs

@@ -2791,7 +2791,7 @@ Interpolating <<gradient>> {#interpolating-gradients}
 
 			* Otherwise, the size must be changed to a pair of <<length>>s
 				that would produce an equivalent ending-shape.
-				If the <<rg-ending-shape>> was specified as <a value spec=css-images-3>circle</a>,
+				If the <<radial-shape>> was specified as <a value spec=css-images-3>circle</a>,
 				change it to ''ellipse''.
 
 	2. Interpolate each component and color-stop of the gradients independently.
@@ -3103,4 +3103,4 @@ Changes Since Level 3</h3>
 <!-- CSS Values 4 test, empty url, on background-image -->
 <wpt hidden>
 	empty-background-image.html
-</wpt>
+</wpt>