[css-font-loading-3] Link to definition of task source, fix w3c#1187

Author: svgeesus

css-font-loading-3/Overview.bs

@@ -60,7 +60,7 @@ Values</h3>
 Task Sources</h3>
 
 	Whenever this specification queues a task,
-	it queues it onto the "font loading" task source.
+	it queues it onto the "font loading" [=task source=].
 
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