Explain better how to set multiple repos for `setup-r`

[msberends]

I had thought this ought to work:

- uses: r-lib/actions/setup-r@v2
  with:
    r-version: ${{ matrix.config.r }}
    use-public-rspm: true
    extra-repositories:
      - 'https://myrepo1.r-universe.dev'
      - 'https://myrepo2.r-universe.dev'

But it doesn't - it should apparently be:

- uses: r-lib/actions/setup-r@v2
  with:
    r-version: ${{ matrix.config.r }}
    use-public-rspm: true
    extra-repositories: 'https://myrepo1.r-universe.dev, https://myrepo2.r-universe.dev'

This was not clear from the documentation, so I updated that.

Still think that the element should be pasted together internally to allow common YAML language like my first example.

[gaborcsardi]

Still think that the element should be pasted together internally to allow common YAML language like my first example.

Sure, a PR is welcome for that. 😄

[msberends]

Still think that the element should be pasted together internally to allow common YAML language like my first example.

Sure, a PR is welcome for that. 😄

Good point, I'll try to familiarise myself with .ts files!

[msberends]

I believe this should do it, but please check critically, I've never worked in TypeScript before.

Should I also update the README.md or will that happen automatically?