[FileAPI] Update the link of RFC2119 (#2)

The link checker complained that the link to RFC2119 "http://www.ietf.org/rfc/rfc2119" results in a redirect loop, that can be fixed by a HTTP Secure URI "https://www.ietf.org/rfc/rfc2119".

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/FileAPI/issues/2