[css-sizing-3] Fix link error.

Author: fantasai

css-sizing-3/Overview.bs

@@ -244,7 +244,7 @@ Auto Box Sizes</h3>
 			derived primarily from one of these two sizes.)
 	</dl>
 
-<h3 id='contributions' dfn export lt="intrinsic size contribution">
+<h3 id='contributions'>
 Intrinsic Size Contributions</h3>
 
 	<dl export>