docs: Update custom fetch example (#329)

Author: jacobwgillespie

README.md

@@ -55,7 +55,9 @@ Then you can use it from a global `supabase` variable:
 import { createClient } from '@supabase/supabase-js'
 
 // Provide a custom `fetch` implementation as an option
-const supabase = createClient('https://xyzcompany.supabase.co', 'public-anon-key', { fetch: fetch })
+const supabase = createClient('https://xyzcompany.supabase.co', 'public-anon-key', {
+  fetch: (...args) => fetch(...args),
+})
 ```
 
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