[css-borders-4] Do not allow a trailing slash (w3c#11639)

Author: cdoublev

css-borders-4/Overview.bs

@@ -410,7 +410,7 @@ Corner Shaping: the 'corner-shape' and 'corner-*-shape' properties</h4>
 
 	<pre class="propdef">
 		Name: corner-shape
-		Value: <<corner-shape-value>>{1,2} / [ <<corner-shape-value>>{1,2} ]?
+		Value: <<corner-shape-value>>{1,2} [ / <<corner-shape-value>>{1,2} ]?
 		Initial: round
 		Applies to: all elements where 'border-radius' can apply
 		Inherited: no