Renaming master branch of Specref to main

Hi folks,

Thanks to Anne reminding me to do this, I just renamed the master branch of
the Specref repo to main.

I've made sure that the deployment pipeline was updated accordingly and
tested it, so you shouldn't notice any changes.

However, mistakes happen, so please file an issue against the repo[1] if
you notice anything weird or if automated deployments get stuck.

I'll tackle doing the same for PR Preview once I'm sure Specref is running
smoothly on main.

Thanks,

--tobie

---
[1]: https://github.com/tobie/specref/issues/new