Sync kit docs (#1658)

sync kit docs

Co-authored-by: svelte-docs-bot[bot] <196124396+svelte-docs-bot[bot]@users.noreply.github.com>
Co-authored-by: ComputerGuy <[email protected]>

Author: 3 people

apps/svelte.dev/content/docs/kit/20-core-concepts/10-routing.md

@@ -320,9 +320,9 @@ By exporting `POST`/`PUT`/`PATCH`/`DELETE`/`OPTIONS`/`HEAD` handlers, `+server.j
 ```svelte
 <!--- file: src/routes/add/+page.svelte --->
 <script>
-	let a = 0;
-	let b = 0;
-	let total = 0;
+	let a = $state(0);
+	let b = $state(0);
+	let total = $state(0);
 
 	async function add() {
 		const response = await fetch('/api/add', {