doc: ensure consistent grammar in node.1 file

PR-URL: https://github.com/nodejs/node/pull/6426


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>