Use data-export attribute for exporting terms

See w3c/webref#560 (comment)

index.html

@@ -951,7 +951,7 @@ <h2>Retargeting</h2>
 
     <section>
       <h2>Conditionally exposing legacy touch event APIs</h2>
-      <p>User agents have an associated boolean <dfn class="export">expose legacy touch event APIs</dfn> whose value is <a>implementation-defined</a>.</p>
+      <p>User agents have an associated boolean <dfn data-export="">expose legacy touch event APIs</dfn> whose value is <a>implementation-defined</a>.</p>
 
       <p class="note">Existing web content often use the existence of these APIs as a signal that the user agent is a touch-enabled "mobile" device, and therefore exposing these APIs on non-mobile devices, even if they are touch-enabled, could lead to a suboptimal user experience for such web content.</p>
     </section>