Fix svelte optimistic updates example (#8005)

Co-authored-by: Jonghyeon Ko <manudeli.ko@gmail.com>

Author: BennettLiam

examples/svelte/optimistic-updates/src/routes/api/data/+server.ts

@@ -19,8 +19,9 @@ export const POST: RequestHandler = async ({ request }) => {
   const { text } = await request.json()
 
   if (Math.random() > 0.7) {
-    json({ message: 'Could not add item!' }, { status: 500 })
+    return json({ message: 'Could not add item!' }, { status: 500 })
   }
+
   const newTodo = {
     id: Math.random().toString(),
     text: text.toUpperCase() as string,