[css-ui] Fix accidental syntax regression (w3c#13018)

[foo,]* and foo#? are not equivalent, as the former has a trailing
comma.

See w3c#13001

Author: frivoal

css-ui-3/Overview.bs

@@ -921,7 +921,7 @@ While the content is being scrolled, implementations may adjust their rendering
 
 <pre class="propdef">
 Name: cursor
-Value: <<cursor-image>>#? <<cursor-predefined>>
+Value: [<<cursor-image>>,]* <<cursor-predefined>>
 Initial: auto
 Applies to: all elements
 Inherited: yes

css-ui-4/Overview.bs

@@ -720,7 +720,7 @@ Styling the Cursor: the 'cursor' property</h4>
 
 	<pre class="propdef">
 	Name: cursor
-	Value: <<cursor-image>>#? <<cursor-predefined>>
+	Value: [<<cursor-image>>,]* <<cursor-predefined>>
 	Initial: auto
 	Applies to: all elements
 	Inherited: yes