docs(api-routes): fix micro-cors example (#9184)

The correct option to use with `micro-cors` package is `allowMethods` instead `allowedMethods`. This PR fixes the example.

packages/next/README.md

@@ -1240,7 +1240,7 @@ Then, import `micro-cors` and [configure it](https://github.com/possibilities/mi
 import Cors from 'micro-cors'
 
 const cors = Cors({
-  allowedMethods: ['GET', 'HEAD'],
+  allowMethods: ['GET', 'HEAD'],
 })
 
 function Endpoint(req, res) {