[css-overflow-3] Fix animation type line

Author: fantasai

css-overflow-3/Overview.bs

@@ -527,6 +527,7 @@ Expanding Clipping Bounds: the 'overflow-clip-margin' property</h3>
 	Inherited: no
 	Applies to: all elements
 	Computed value: the computed <<length>> and a <<visual-box>> keyword
+	Animation type: per computed value if the <<visual-box>> values match; otherwise discrete
 	</pre>
 
 	This property defines the <dfn export>overflow clip edge</dfn> of the box,