doc: unlock module

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

PR-URL: nodejs/node#11661
Ref: nodejs/node#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>

doc/api/modules.md

@@ -1,6 +1,6 @@
 # Modules
 
-> Stability: 3 - Locked
+> Stability: 2 - Stable
 
 <!--name=module-->