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>

lib/module.js

@@ -632,7 +632,7 @@ Module._initPaths = function() {
 
   modulePaths = paths;
 
-  // clone as a read-only copy, for introspection.
+  // clone as a shallow copy, for introspection.
   Module.globalPaths = modulePaths.slice(0);
 };