doc: rename iojs-* groups to nodejs-*

As per #2525 a bunch of WGs are renamed from iojs-* to nodejs-*.
Update the WORKING_GROUPS.md to match.

Note specifically iojs-cn and iojs-tw were renamed to nodejs-zh-CN and
nodejs-zh-TW respectively.

Fixes: https://github.com/nodejs/node/issues/3247
PR-URL: https://github.com/nodejs/node/pull/3634


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