doc: fix typos and sources in WORKING_GROUPS.md

`therefor` is a typo of `therefore`, and was fixed. There were also two
places where the website WG was directly linked, where they should have
put the WG's name/repo; that was fixed as well.

PR-URL: https://github.com/iojs/io.js/pull/1022


Reviewed-By: Mikeal Rogers <mikeal.rogers@gmail.com>
Reviewed-By: Christian Tellnes <christian@tellnes.no>