[css-images-4] Fix escaping in object-view-box definition (w3c#7113)

Author: tidoust

css-images-4/Overview.bs

@@ -1936,7 +1936,7 @@ Setting The Viewbox: the 'object-view-box' property {#the-object-view-box}
 
 	<pre class=propdef>
 	Name: object-view-box
-	Value: none | <basic-shape-rect>
+	Value: none | <<basic-shape-rect>>
 	Initial: none
 	Applies to: [=replaced elements=]
 	Inherited: no