Correctly display elapsed in the tutorial example

Otherwise people learning svelte will see a `This page has been open for undefined seconds`

site/content/tutorial/08-stores/04-derived-stores/app-a/stores.js

@@ -14,5 +14,5 @@ const start = new Date();
 
 export const elapsed = derived(
 	time,
-	$time => {}
-);
+	$time => Math.round(($time - start) / 1000)
+);