add SubmitErrorHandler type to handleSubmit example

Author: aken-you

src/content/docs/useform/handlesubmit.mdx

@@ -56,7 +56,7 @@ This function will receive the form data if form validation is successful.
 
 ```typescript copy sandbox="https://codesandbox.io/s/react-hook-form-handlesubmit-ts-v7-lcrtu"
 import React from "react"
-import { useForm, SubmitHandler } from "react-hook-form"
+import { useForm, SubmitHandler, SubmitErrorHandler } from "react-hook-form"
 
 type FormValues = {
   firstName: string
@@ -67,6 +67,7 @@ type FormValues = {
 export default function App() {
   const { register, handleSubmit } = useForm<FormValues>()
   const onSubmit: SubmitHandler<FormValues> = (data) => console.log(data)
+  const onError: SubmitErrorHandler<FormValues> = (errors) => console.log(errors)
 
   return (
     <form onSubmit={handleSubmit(onSubmit)}>