docs: onRequestError request header type (#83988)

Aligning with the actual type
https://github.com/vercel/next.js/blob/canary/packages/next/src/server/instrumentation/types.ts#L18

Author: icyJoseph

docs/01-app/03-api-reference/03-file-conventions/instrumentation.mdx

@@ -88,7 +88,7 @@ export function onRequestError(
   request: {
     path: string // resource path, e.g. /blog?name=foo
     method: string // request method. e.g. GET, POST, etc
-    headers: { [key: string]: string }
+    headers: { [key: string]: string | string[] }
   },
   context: {
     routerKind: 'Pages Router' | 'App Router' // the router type