Provide service_account example in values.yaml to reduce confusion

What does this MR do?

We have a lot of customers running into Impossible to customize the Kubernetes Service Account used by the gitlab-runner pods..

There is a simple solution in this comment to add service_account = "" into the Kubernetes runner config section.

Although the solution is simple, it appears that many customers and users struggle to find it.

Why was this MR needed?

Our customer in https://gitlab.zendesk.com/agent/tickets/519822 (Internal only) advised that we should put a reference to service_account in our default values as it would have saved them a lot of time.