Correct custom-instance-setup script invocation

What does this MR do?

The custom-instance-setup script was changed from a bash script to a ruby script in gitlab-org/build/CNG!590 (merged) but the instructions for manually running it as part of a restore of non-chart backup into a chart deployment were not updated to take account of this.

Result of currently documented command:

$ kubectl exec -it -n gitlab gitlab-task-runner-7959f4f89d-vrhrb -- /scripts/custom-instance-setup
Defaulted container "task-runner" out of: task-runner, certificates (init), configure (init)
Disabling authorized keys write in the database.
Traceback (most recent call last):
/scripts/custom-instance-setup:7:in `<main>': uninitialized constant ApplicationSetting (NameError)
command terminated with exit code 1

Result of proposed updated command:

$ kubectl exec -it -n gitlab gitlab-task-runner-7959f4f89d-vrhrb -- gitlab-rails runner -e production /scripts/custom-instance-setup
Defaulted container "task-runner" out of: task-runner, certificates (init), configure (init)
Disabling authorized keys write in the database.
Enabling incremental logging of CI jobs.
Disabling access to disk storage for GitLab Pages
Registering OAuth applications.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: ~"feature", frontend, backend, ~"bug", or database

Review checklist

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