doc: fix Folders as Modules omission of index.json

This section was inconsistent with the actual behavior of `require` and
with other parts of this same document, which do describe the ability
to treat index.json as a module’s default file.

PR-URL: https://github.com/joyent/node/pull/8868


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>