Update docs after branch rename `master` -> `main`

[Caiopia]

This just updates the file paths after renaming master to main. It's probably worth checking if those links still go to the right spots.

[Caiopia]

You can test the file paths by checking the branch out and opening the docs locally at file://<path to FunctionalTableData>/docs/index.html. Next, navigate to a function or type that offers the option to "View on GitHub" and verify it doesn't 404 anymore compared with the hosted docs https://shopify.github.io/FunctionalTableData/.