doc: use `Node.js` in synopsis document

Change an instance of `Node` in the synopsis document to `Node.js.`

PR-URL: https://github.com/nodejs/node/pull/6476


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>