Clarify the offline steps for the AI Gateway Helm chart

What does this MR do?

The LITELLM_LOCAL_MODEL_COST_MAP sentence now says True, as the command sets it. Either spelling works with --set-string, because LiteLLM 1.101.0, which the self-hosted-v19.4.1-ee image pins, compares the value case-insensitively.

The mirror step now sets grpc-tls-proxy.image.repository, which defaults to nginx in the TLS proxy subchart.

Prepared with help from Claude Code and checked against source and the documentation style guide. Vale at suggestion level reports no new alerts on the changed lines.

Related issues

Follow-up to !257999 (merged), from its GitLab Duo review.

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