Fix outdated link for Font Awesome icon set

Summary:
The current link has a redirect for a while now, from
http://fortawesome.github.io/Font-Awesome/ to https://fontawesome.com

However, since the release of Version 5, the docs no longer
match the icons that are valid for use in Phabricator, which
uses Version 4.

Update the reference to link to the same logical content as before.

Test Plan: The content now lives at https://fontawesome.com/v4.7.0/icons/.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D19241