doc: fix typo in esm.md

_subpath_ is not defined in this context. This is pretty clearly meant
to be _packageSubpath_, which is the second argument to
`PACKAGE_SELF_RESOLVE`

PR-URL: https://github.com/nodejs/node/pull/40273


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>