Fix incorrect links in spec index

[elichad]

Fixes #169 in 1.1, 1.2, and the template (1.3-DRAFT), with the exception of:

• single-page HTML is a reference to https://github.com/ResearchObject/ro-crate/releases/download/1.2.0/ro-crate-1.2.0.html which returns a Location header to an asset in github causing the content to be returned as an octet-stream rather than rendered directly ( that may be intended but was not expected behaviour )

I'm not sure if anything should change there since we don't host the single-page HTML on the website.