Docs: Update autoscale_aws_fargate to include ca certificate location

What does this MR do?

This MR updates docs/configuration/runner_autoscale_aws_fargate/index.md to include ca certificate location in the example:

[runners.custom]
       volumes = ["/cache", "/path/to-ca-cert-dir/ca.crt:/etc/gitlab-runner/certs/ca.crt:ro"]

Self-managed customers, who use a private CA will need to add this line in order for the Fargate runner to work appropriately under the custom executor. Having this added to the fargate example helps avoid this oversight.

Related issues

https://gitlab.zendesk.com/agent/tickets/183258

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• Follow the Documentation Guidelines and Style Guide.
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• Update the permissions table.
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