docs: fix esm loader options in ESM stubbing guide

[alorentiar]

The loader example in this guide passes mutableNamespace as a top-level
esm option:

require = require("esm")(module, {
  cjs: true,
  mutableNamespace: true,
});

esm only reads it under cjs (see the options table in its README), and it
rejects unknown top-level keys, so this loader actually fails on startup with
Unknown esm@3.2.25 option: mutableNamespace. I reproduced that with mocha +
esm on an older Node (esm only runs there), then confirmed the corrected
options load cleanly.

Moved the flag under cjs in both places it appears. Keeping it scoped to
the reported config issue.

Refs #2714