Update configuring_file_domain.diviner's cloudflare link

Summary: https://cloudflare.net generates SSL certification error, and even if I click "ignore it", the page is 403 forbidden afterwards. Their domain has moved to .com, therefore update it to reflect this.

Test Plan:
Go to Configuring a File Domain, click CloudFlare.
Then, apply the patch. Refresh the page, and the new link works.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: revi, epriestley

Differential Revision: https://secure.phabricator.com/D14172