[web-animations-1] Scope export of default document timeline

Fixes w3c#8002.

Author: birtles

web-animations-1/Overview.bs

@@ -750,7 +750,7 @@ time=]. If |timeline| is inactive, return an [=unresolved=] [=time value=].
 ### The default document timeline ### {#the-documents-default-timeline}
 
 Each {{Document}} has a <a>document timeline</a> called the
-<dfn export>default document timeline</dfn>.
+<dfn export for="timeline">default document timeline</dfn>.
 The <a>default document timeline</a> is unique to each document and persists for
 the lifetime of the document including calls to <a>document.open()</a> [[!HTML]].