[docs] added transition on toggling folder in tutorial (#6453)

site/content/examples/17-special-elements/00-svelte-self/Folder.svelte

@@ -1,5 +1,6 @@
 <script>
 	import File from './File.svelte';
+	import {slide} from 'svelte/transition'
 	
 	export let expanded = false;
 	export let name;
@@ -13,7 +14,7 @@
 <span class:expanded on:click={toggle}>{name}</span>
 
 {#if expanded}
-	<ul>
+	<ul transition:slide={{duration:300}}>
 		{#each files as file}
 			<li>
 				{#if file.type === 'folder'}