[css-multicol] Export dfn.

Author: tabatkins

css-multicol/Overview.bs

@@ -165,7 +165,7 @@ Introduction</h2>
 <h2 id="the-multi-column-model">
 The multi-column model</h2>
 
-	A <dfn lt="multi-column element|multicol element">multi-column element</dfn>
+	A <dfn lt="multi-column element|multicol element" export>multi-column element</dfn>
 	(or <em>multicol element</em> for short)
 	is an element whose 'column-width' or 'column-count'
 	property is not ''column-width/auto'' and therefore acts as a container for