Broken link

[mzeinstra]

The link in section 3.5 on RFC 3339 on https://c2pa.org/specifications/specifications/2.1/specs/C2PA_Specification.html#_embeddable_formats is broken

It links to:

https://https//www.rfc-editor.org/rfc/rfc3339

but should be

https://www.rfc-editor.org/rfc/rfc3339

Fix in #87