doc: fix markdown formatting in url.md

Tangentially related to https://github.com/nodejs/node/pull/7817

PR-URL: https://github.com/nodejs/node/pull/8933


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>