clarify which cluster k8s executor is referring to

What does this MR do?

Clarifies which cluster we're referring to in the docs for k8s executor.

Why was this MR needed?

Either I don't understand it well or it's ambiguous a bit atm.

What's the best way to test this MR?

it's docs - find knowledgable person to confirm 😅

What are the relevant issue numbers?

none