fix: Fix shape tree urls

[sverholen]

Not sure if it's ok to add the Shape Tree specification as a normative reference, since it's only an editor's draft, but the URL was wrong: https://shapetrees.github.io/specification/spec , changed it to https://shapetrees.org/TR/specification/

Other candidates:

• https://shapetrees.org/
• https://github.com/shapetrees/
• https://github.com/shapetrees/specification
• Primer: https://shapetrees.org/TR/primer/

[pietercolpaert]

Thanks!