[css-images] Fix image-rendering grammar to align with prose

Author: LeaVerou

css-images/Overview.bs

@@ -1488,7 +1488,7 @@ Determing How To Scale an Image: the 'image-rendering' property {#the-image-rend
 
 	<pre class='propdef'>
 	Name: image-rendering
-	Value: auto | crisp-edges | pixelated
+	Value: auto | smooth | high-quality | crisp-edges | pixelated
 	Initial: auto
 	Applies to: all elements
 	Inherited: yes