docs:Update App.svelte in Clock example according new rune syntax (#1127)

Update App.svelte

Author: pochemuto

apps/svelte.dev/content/examples/12-svg/01-clock/+assets/App.svelte

@@ -4,7 +4,7 @@
 	let time = $state(new Date());
 
 	// these automatically update when `time`
-	// changes, because of the `$:` prefix
+	// changes, because of the $derived
 	let hours = $derived(time.getHours());
 	let minutes = $derived(time.getMinutes());
 	let seconds = $derived(time.getSeconds());