docs: remove this usage from function component (#29596)

Fix a small error in the `next/script` docs.

Let me know if you'd like a `useState` for the sake of a complete example.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes

docs/basic-features/script.md

@@ -109,16 +109,19 @@ export default function Home() {
 ### Executing Code After Loading (`onLoad`)
 
 ```js
+import { useState } from 'react'
 import Script from 'next/script'
 
 export default function Home() {
+  const [stripe, setStripe] = useState(null)
+
   return (
     <>
       <Script
         id="stripe-js"
         src="https://js.stripe.com/v3/"
         onLoad={() => {
-          this.setState({ stripe: window.Stripe('pk_test_12345') })
+          setStripe({ stripe: window.Stripe('pk_test_12345') })
         }}
       />
     </>