Update custom-error-page for getServerSideProps (#41723)

`fetch` output type has `status` but no `statusCode`, see MDN https://developer.mozilla.org/en-US/docs/Web/API/Response/status


## Documentation / Examples

- [x] Make sure the linting passes by running `pnpm lint`
- [x] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

docs/advanced-features/custom-error-page.md

@@ -74,7 +74,7 @@ import Error from 'next/error'
 
 export async function getServerSideProps() {
   const res = await fetch('https://api.github.com/repos/vercel/next.js')
-  const errorCode = res.ok ? false : res.statusCode
+  const errorCode = res.ok ? false : res.status
   const json = await res.json()
 
   return {