Fix typo: Microsoft Sentienl -> Microsoft Sentinel

[changeworld]

[prmerger-automator[bot]]

@changeworld : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 11f1537:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/sentinel/billing-monitor-costs.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

[v-ccolin]

I sent an email to the content owner today.

@MicrosoftDocs/public-repo-pr-review-team