ci: update actions/checkout from v2 to v3 in example

[mcanouil]

This PR updates the GitHub Action "checkout" from v2 to v3.
See https://github.com/actions/checkout.

[cderv]

Thanks

[fuhrmanator]

I also updated my (copy) of this example because I was getting warnings about node 12 being deprecated. But I was surprised to see I was still getting warnings about using checkout@v2 (node 12). After sleuthing, I discovered that GitHub's own build action (that puts stuff on GitHub pages) is still not updated! See more at https://github.com/orgs/community/discussions/37224