[css-syntax] Add a dfn for serializaing An+B.

Author: tabatkins

css-syntax/Overview.bs

@@ -3372,7 +3372,7 @@ Serialization</h2>
 <h3 id='serializing-anb'>
 Serializing <var>&lt;an+b></var></h3>
 
-	To serialize an <var>&lt;an+b></var> value,
+	To <dfn export>serialize an <var>&lt;an+b></var> value</dfn>,
 	let <var>s</var> initially be the empty string:
 
 	<dl>