doc: fix file extension on ESM file example

PR-URL: #25692
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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
 ```