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

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

@@ -1,6 +1,7 @@
 <script>
 	import File from './File.svelte';
-
+	import {slide} from 'svelte/transition'
+	
 	export let expanded = false;
 	export let name;
 	export let files;
@@ -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'}
@@ -49,4 +50,4 @@
 	li {
 		padding: 0.2em 0;
 	}
-</style>
+</style>