module: fix comment from "read-only" to "shallow"

The comment here was misleading, implying that the property was being
copied as a read-only, when in fact it's just a shallow copy. This
serves the purpose of providing the array for introspection, but it
isn't read-only.

PR-URL: https://github.com/nodejs/node/pull/8887


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>