README.md uses incorrect variable names

[ncthbrt]

From the source code, resourceGroupName should actually be resource-group,
while clusterName should actually be cluster-name

[benc-uk]

Just found the same :(

[ncthbrt]

Pinging @DS-MS, who seems to be the main contributor to the this repo:
This Github action is likely numbered one of the first that Azure Aks users evaluating Github actions is going to try out, at least it was for me, so it kind of feels like a bad look that the example given doesn't actually work.

[thesattiraju]

Thanks for the PR. I missed updating the readme with some last minute changes we did.

I've merged it. I'll update this in the marketplace as well.

[ncthbrt]

Thanks a lot!