doc: reorder collaborators by their usernames

Fixes: https://github.com/nodejs/node/issues/1972
PR-URL: https://github.com/nodejs/node/pull/2322


Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: thefourtheye - Sakthipriyan Vairamani <thechargingvolcano@gmail.com>