Docs: Suggest single command to obtain K8S cert

What does this MR do?

Condenses the Kubernetes certificate instructions into a single Bash command, as for the API URL in the step above it and for gitlab-admin below it. Noticing this little inconsistency during onboarding training, I'm hoping this update is useful.

cc @jcolyer & @joshlambert, because you introduced this in 54330656 & 2d16f479, respectively. In case there was a particular reason to have this in 2 steps, please feel free to close this MR.

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