[css-multicol-2] Fix markup typo

Author: frivoal

css-multicol-2/Overview.bs

@@ -158,7 +158,7 @@ Spanning columns</h2>
 
 			Issue: Add final content from previous level
 
-		<dt><dft><<integer>></dfn>
+		<dt><dfn><<integer>></dfn>
 		<dd>
 			The element spans the specified number of columns.
 			Values must be greater than 0.