Use correct branch as the basis for deploying the docs

[engineervix]

When the GitHub Actions configuration for the docs was initially defined, we had master as the default branch. We have since switched to main, but the configuration still maintained master. This PR corrects this.

How to Test

We can only test this once we merge it to the main branch. I have made a rather minor edit to the docs (cae9aa9) just to trigger a GitHub Pages build when this PR is merged.

Note

a GitHub Pages build can also be invoked by running the Docs workflow manually from the Actions tab

MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• Tests and linting passes.
• Consider updating documentation. If you don't, tell us why.
• If relevant, list the environments / browsers in which you tested your changes.

[helenb]

LGTM thanks @engineervix - I'll make the same amendments in the new repo