Fix gitlab-runner config values documentation
This fixes the service vs services spelling in the doc, and adds missing helpers.* values, as per latest state in https://gitlab.com/gitlab-org/charts/gitlab-runner/-/blob/master/values.yaml#L258
This fixes the service vs services spelling in the doc, and adds missing helpers.* values, as per latest state in https://gitlab.com/gitlab-org/charts/gitlab-runner/-/blob/master/values.yaml#L258