docs:Update App.svelte in Clock example according new rune syntax

[pochemuto]

Current example doesn't have legacy "$:" syntax. The comment is misleading.

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Name	Status	Preview	Updated (UTC)
svelte-dev	❌ Failed (Inspect)		Feb 18, 2025 7:06pm

[Conduitry]

Do we want to rework this a bit further and use https://svelte.dev/tutorial/svelte/reactive-builtins instead?

[Rich-Harris]

We actually don't gain a lot by doing so in this case — the diff looks like this:

<script>
	import { onMount } from 'svelte';
+	import { SvelteDate } from 'svelte/reactivity';

-	let time = $state(new Date());
+	let time = $state(new SvelteDate());

	// these automatically update when `time`
	// changes, because of the `$:` prefix
	let hours = $derived(time.getHours());
	let minutes = $derived(time.getMinutes());
	let seconds = $derived(time.getSeconds());

	onMount(() => {
		const interval = setInterval(() => {
-			time = new Date();
+			time.setTime(Date.now());
		}, 1000);

		return () => {
			clearInterval(interval);
		};
	});
</script>

So I'll merge this as-is — thanks!