docs: avoid legacy Svelte syntax (#13166)

* avoid invalid svelte syntax

* type submit event correctly

---------

Co-authored-by: Harry Allen <[email protected]>

Author: HarryAllen1

documentation/docs/20-core-concepts/30-form-actions.md

@@ -430,8 +430,9 @@ We can also implement progressive enhancement ourselves, without `use:enhance`,
 	/** @type {{ form: import('./$types').ActionData }} */
 	let { form } = $props();
 
-	/** @param {{ currentTarget: EventTarget & HTMLFormElement}} event */
+	/** @param {SubmitEvent & { currentTarget: EventTarget & HTMLFormElement}} event */
 	async function handleSubmit(event) {
+		event.preventDefault();
 		const data = new FormData(event.currentTarget);
 
 		const response = await fetch(event.currentTarget.action, {
@@ -451,7 +452,7 @@ We can also implement progressive enhancement ourselves, without `use:enhance`,
 	}
 </script>
 
-<form method="POST" onsubmit|preventDefault={handleSubmit}>
+<form method="POST" onsubmit={handleSubmit}>
 	<!-- content -->
 </form>
 ```