docs: make awaited function async

Author: Just-Moh-it

README.md

@@ -434,7 +434,7 @@ import { fetch } from 'undici'; // as one example
 import OpenAI from 'openai';
 
 const client = new OpenAI({
-  fetch: (url: RequestInfo, init?: RequestInfo): Response => {
+  fetch: async (url: RequestInfo, init?: RequestInfo): Response => {
     console.log('About to make request', url, init);
     const response = await fetch(url, init);
     console.log('Got response', response);