Update Carmen link to docs

[kalbaxa]

This updates the link to Carmen GeoJSON reference to lead to the docs page because carmen repo is private now.

• [x] briefly describe the changes in this PR
• [ ] write tests for all new functionality
• [ ] run npm run docs and commit changes to API.md
• [ ] update CHANGELOG.md with changes under master heading before merging

cc @mikelmaron @a-s8h