event/TimerEvent: update API documentation

src/event/TimerEvent.hxx

@@ -46,7 +46,7 @@ class TimerEvent final
 	const Callback callback;
 
 	/**
-	 * When is this timer due?  This is only valid if IsActive()
+	 * When is this timer due?  This is only valid if IsPending()
 	 * returns true.
 	 */
 	Event::Clock::time_point due;