docs: update custom fetch example (#235)

Author: jacobwgillespie

README.md

@@ -38,7 +38,9 @@ const postgrest = new PostgrestClient(REST_URL)
 import { PostgrestClient } from '@supabase/postgrest-js'
 
 const REST_URL = 'http://localhost:3000'
-const postgrest = new PostgrestClient(REST_URL, { fetch: fetch })
+const postgrest = new PostgrestClient(REST_URL, {
+  fetch: (...args) => fetch(...args),
+})
 ```
 
 ## License