doc: add useState for handleError and setDisabled

src/Checkout/Payment/Payment.js

@@ -1,4 +1,4 @@
-import React from "react";
+import React, { useState } from "react";
 import { useStateValue } from "../../redux/StateProvider";
 import { Link } from "react-router-dom";
 import CheckoutProduct from "../CheckoutProduct";
@@ -10,6 +10,9 @@ function Payment() {
   const stripe = useStripe();
   const elements = useElements();
 
+  const [error, handleError] = useState(null);
+  const [disabled, setDisabled] = useState(true);
+
   const handleSubmit = (e) => {};
   const handleChange = (e) => {};