[mediaqueries-5] Export the dfn for display mode

Part of w3c#6343

Author: frivoal

mediaqueries-5/Overview.bs

@@ -1305,7 +1305,7 @@ Display Modes: the ''display-mode'' media feature </h3>
 	The 'display-mode' media feature represents the [=display mode=] of the web application.
 	Child browsing contexts reflect the [=display mode=] of their [=top-level browsing context=].
 
-	A <dfn>display mode</dfn> represents
+	A <dfn export>display mode</dfn> represents
 	how the web application is being presented within the context of an OS (e.g., in fullscreen, etc.).
 	Display modes correspond to user interface (UI) metaphors
 	and functionality in use on a given platform.