More correct syntax

More correct and relevant syntax, so people don't confuse it with event.target (which is basically same as event.detail) when using TypeScript

site/content/tutorial/05-events/04-component-events/app-b/App.svelte

@@ -2,7 +2,7 @@
 	import Inner from './Inner.svelte';
 
 	function handleMessage(event) {
-		alert(event.detail.text);
+		alert(event.target.text);
 	}
 </script>