Update email subject example to use first alert target

[xuegao112]

Updated the email subject example in the alert customization documentation to use the correct syntax for array fields in the common alert schema. Replaced ${data.essentials.alertTargetIDs} with ${data.essentials.alertTargetIDs[0]}, since alertTargetIDs is an array and must be accessed with an index.

[prmerger-automator]

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[learn-build-service-prod]

Learn Build status updates of commit cf25588:

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File	Status	Preview URL	Details
articles/azure-monitor/alerts/alerts-customize-email-subject-how-to.md	✅Succeeded

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[Copilot]

Pull Request Overview

This PR fixes an incorrect email subject example in the Azure Monitor alert customization documentation by properly accessing array elements in the common alert schema.

• Updated email subject template to use proper array indexing syntax
• Corrected alertTargetIDs reference to access the first element with [0]

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[ttorble]

@xuegao112 did you intend to open this PR in the private repo? It looks like you have access to the private repo, and if so, you should use that for content updates. I'll leave this PR open in case @IngridAtMicrosoft wants to review it here. If you decide to open a new PR in the private repo, please close this PR. Thanks.

@IngridAtMicrosoft

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team