docs/file_domain: Update cloudflare spelling and URLs

Summary:
Cloudflare now spell themselves as Cloudflare.[1]

[1]: https://www.cloudflare.com/trademark/

Signed-off-by: Yongmin Hong <revi@omglol.email>

Test Plan: Read the Docs ™️.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25616