docs(form): improve joi example

docs/app/components/content/examples/form/FormExampleJoi.vue

@@ -15,7 +15,7 @@ const state = reactive({
 })
 
 const toast = useToast()
-async function onSubmit(event: FormSubmitEvent<typeof schema>) {
+async function onSubmit(event: FormSubmitEvent<typeof state>) {
   toast.add({ title: 'Success', description: 'The form has been submitted.', color: 'success' })
   console.log(event.data)
 }