Fix param name in terminate_indexer_worker task

[stacimc]

Fixes

Fixes #[issue number] by @[issue author]

Description

Fixes the param name to terminate_instances, matching documentation.

Testing Instructions

This cannot be tested locally; once merged, we'll test manually on the staging data refresh DAG.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
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• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

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