docs: add missing fat arrow (vercel#4245)

Author: gokaygurcan

readme.md

@@ -1128,7 +1128,7 @@ Phases can be imported from `next/constants`:
 
 ```js
 const {PHASE_DEVELOPMENT_SERVER} = require('next/constants')
-module.exports = (phase, {defaultConfig}){
+module.exports = (phase, {defaultConfig}) => {
   if(phase === PHASE_DEVELOPMENT_SERVER) {
     return {
       /* development only config options here */