doc: s/node.js/Node.js in readme

PR-URL: https://github.com/nodejs/node/pull/3998


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>