Fix next.config.mjs constants.js import

Author: davidde

LEARNING.md

@@ -7,7 +7,7 @@ By default, Next.js uses Node.js to run the application, which is incompatible w
 
 Update `next.config.mjs` with the following:
 ```mjs
-import { PHASE_PRODUCTION_SERVER } from 'next/constants';
+import { PHASE_PRODUCTION_SERVER } from 'next/constants.js';
 
 export default (phase) => {
   const isProd = phase === PHASE_PRODUCTION_SERVER;

next.config.mjs

@@ -1,4 +1,4 @@
-import { PHASE_PRODUCTION_SERVER } from 'next/constants';
+import { PHASE_PRODUCTION_SERVER } from 'next/constants.js';
 
 export default (phase) => {
   const isProd = phase === PHASE_PRODUCTION_SERVER;