Config docs: Fix syntax in example TOML for Kubernetes

Unlike YAML, indentation in TOML has no meaning. All key-value pairs that follow a table [header] will go inside that table, regardless of how they are indented.

The example for the kubernetes executor configuration looked like this:

[runners.kubernetes]
  # ...
  privileged = true
  [runners.kubernetes.node_selector]
    gitlab = "true"
  image_pull_secrets = ["docker-registry-credentials"]

But this will put image_pull_secrets inside the node_selector table, while the documentation says image_pull_secrets is a property of runners.kubernetes.

Fixed by moving image_pull_secrets before the node_selector table.