doc: fix process.stdout reference to console.log

This commit changes the word introduction to the code block to be
explicitly correct, and changes the `d` variable to `msg` for clarity.

PR-URL: https://github.com/iojs/io.js/pull/964


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Nicu Micleușanu <micnic90@gmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>