[css-shapes-1] Use opacity-value (the value) not opacity (the property) as value of shape-image-threshold. Fix w3c#8311

Author: svgeesus

css-shapes-1/Overview.bs

@@ -946,7 +946,7 @@ Choosing Image Pixels: the 'shape-image-threshold' property</h3>
 
 	<pre class='propdef'>
 		Name: shape-image-threshold
-		Value: <<'opacity'>>
+		Value: <<opacity-value>>
 		Initial: 0
 		Applies to: floats
 		Inherited: no