Fix Slack message formatting for ES health alert

[AetherUnbound]

Description

Small issue I noticed while we've been receiving ES health Slack alerts:

Since Slack forming isn't actually markdown, the double asterisk ** was showing up as unformatted when the message should have had bold text. This PR corrects the instances of ** to use a single * instead which will bold the message in Slack.

Testing Instructions

Tests should pass.

You can also observe this locally by running the following (as a maintainer):

1. just c i (to start the catalog & ingestion server resources)
2. j run webserver variables set SLACK_MESSAGE_OVERRIDE true (to enable sending the message to Slack)
3. Enable the staging_elasticsearch_cluster_healthcheck DAG

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• 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 (just catalog/generate-docs for catalog
  PRs) or the media properties generator (just catalog/generate-docs media-props
  for the catalog or just api/generate-docs for the API) where applicable.

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[zackkrida]

Nice!

[obulat]

Great catch!

[dhruvkb]

**LGTM**