doc: unlock module

Update documentation for `module` to reflect stability index 2 (Stable)
rather than 3 (Locked).

PR-URL: https://github.com/nodejs/node/pull/11661
Ref: https://github.com/nodejs/node/issues/11200


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>