[css-color-5] Add missing groups in <light-dark-image> syntax

Follow-up on 100146f.

Author: cdoublev

css-color-5/Overview.bs

@@ -2795,7 +2795,7 @@ or any other color or monochrome output device which has been characterized.
 	<pre class='prod'>
 		<dfn export>light-dark()</dfn> = <<light-dark-color>> | <<light-dark-image>>
 		<dfn export><<light-dark-color>></dfn> = light-dark(<<color>>, <<color>>)
-		<dfn export><<light-dark-image>></dfn> = light-dark(<<image>> | none, <<image>> | none) 
+		<dfn export><<light-dark-image>></dfn> = light-dark( [ <<image>> | none ] , [ <<image>> | none ] ) 
 	</pre>
 
 	For the color form, this function computes to the computed value of the first color,