[css-sizing-3] Fix markup

Author: fantasai

css-sizing-3/Overview.bs

@@ -268,7 +268,7 @@ Intrinsic Size Contributions</h3>
 		<dd>
 			The size that a box contributes to its <a>containing block</a>'s <a>min-content size</a>.
 
-		<dd><dfn>intrinsic size contribution</dfn>
+		<dt><dfn>intrinsic size contribution</dfn>
 		<dd>
 			A <a>max-content contribution</a>, <a>min-content contribution</a>,
 			or similarly-calculated content-based size contribution.