doc: fix file extension on ESM file example

doc/api/esm.md

@@ -98,7 +98,7 @@ representing the value of `module.exports` at the time they finished evaluating.
 // foo.js
 module.exports = { one: 1 };
 
-// bar.js
+// bar.mjs
 import foo from './foo.js';
 foo.one === 1; // true
 ```