doc: corrected name of argument in socket.send

Corrected name of "msg" argument from "buf" to "msg".

PR-URL: https://github.com/nodejs/node/pull/5449


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>