Update usage example in readme

[SeanKilleen]

To reflect current usage.

[berezovskyi]

Thanks for continuing the maintenance of this action!

I just succeeded with https://github.com/oslc4net/oslc4net.github.io/blob/2b7618c4b72ea47a6fb5975a8819f5018f872005/.github/workflows/docfx.yaml#L50-L74 configuration, using https://github.com/actions/deploy-pages over maxheld83/ghpages (just as maxheld83 recommended).

Is that the kind of an update you are looking for?

[SeanKilleen]

Thanks! This was a reminder for me to update the https://github.com/nunit/docfx-action/blob/master/README.md contents to ensure they reflected current usage.

If you'd like, feel free to edit the README.md example in this repo to document that usage and I'd be happy to accept the PR! If not, still thanks for the example. :)