doc: fix file name to main.mjs and not main.js in esm.md

PR-URL: #34786
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/esm.md

@@ -1438,8 +1438,8 @@ console.log(VERSION);
 
 With this loader, running:
 
-```console
-node --experimental-loader ./https-loader.mjs ./main.js
+```bash
+node --experimental-loader ./https-loader.mjs ./main.mjs
 ```
 
 Will print the current version of CoffeeScript per the module at the URL in