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

PR-URL: https://github.com/nodejs/node/pull/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>