Fix error in api-routes-middleware example (#62788)

## What?

Currently it shows `TypeError [ERR_INVALID_ARG_TYPE]: The "string"
argument must be of type string or an instance of Buffer or
ArrayBuffer.`. It just needs to `JSON.stringify` the value as it can be
an array.

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Closes NEXT-2674

examples/api-routes-middleware/pages/api/cookies.ts

@@ -5,5 +5,5 @@ export default function handler(_req: NextApiRequest, res: NextApiResponse) {
   // Calling our pure function using the `res` object, it will add the `set-cookie` header
   setCookie(res, "Next.js", "api-middleware!");
   // Return the `set-cookie` header so we can display it in the browser and show that it works!
-  res.end(res.getHeader("Set-Cookie"));
+  res.end(JSON.stringify(res.getHeader("Set-Cookie")));
 }