[css-images-4] Allow omitting resolution/type in image-set()

Author: cdoublev

css-images-4/Overview.bs

@@ -189,7 +189,7 @@ Resolution/Type Negotiation: the ''image-set()'' notation {#image-set-notation}
 	<pre class='prod'>
 	<<image-set()>> = image-set( <<image-set-option>># )
 	<dfn>&lt;image-set-option></dfn> = [ <<image>> | <<string>> ]
-	                     [ <<resolution>> || type(<<string>>) ]
+	                     [ <<resolution>> || type(<<string>>) ]?
 	</pre>
 
 	Issue: We should add "w" and "h" dimensions as a possibility