docs: update Keyed Each Blocks tutorial

align with sveltejs/svelte#8188

Author: dummdidumm

content/tutorial/01-svelte/04-logic/05-keyed-each-blocks/app-a/src/lib/Thing.svelte

@@ -1,4 +1,6 @@
 <script>
+	import { onDestroy } from 'svelte';
+
 	const emojis = {
 		apple: '🍎',
 		banana: '🍌',
@@ -12,6 +14,11 @@
 
 	// ...but the "emoji" variable is fixed upon initialisation of the component
 	const emoji = emojis[name];
+
+	// observe in the console which entry is removed
+	onDestroy(() => {
+		console.log('thing destroyed: ' + name);
+	});
 </script>
 
 <p>

content/tutorial/01-svelte/04-logic/05-keyed-each-blocks/app-b/src/lib/Thing.svelte

@@ -1,4 +1,6 @@
 <script>
+	import { onDestroy } from 'svelte';
+
 	const emojis = {
 		apple: '🍎',
 		banana: '🍌',
@@ -12,6 +14,11 @@
 
 	// ...but the "emoji" variable is fixed upon initialisation of the component
 	const emoji = emojis[name];
+
+	// observe in the console which entry is removed
+	onDestroy(() => {
+		console.log('thing destroyed: ' + name);
+	});
 </script>
 
 <p>