Update how-to-azure-ad.md (Typo "flexible server"→"Flexible Server")

[hyoshioka0128]

https://github.com/MicrosoftDocs/azure-databases-docs/blob/main/articles/mysql/flexible-server/how-to-azure-ad.md
#PingMSFTDocs

[prmerger-automator]

@hyoshioka0128 : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[hyoshioka0128]

@SudheeshGH - related #10

[learn-build-service-prod]

Learn Build status updates of commit 4503f99:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/mysql/flexible-server/how-to-azure-ad.md	✅Succeeded

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[Jak-MS]

#label:"missed-content-complete"
#label:"event-triaged-to-10AM"

This PR is ready for merge. Due to the upcoming Ignite conference, all merging and publishing is on hold until the morning of the keynote address.

The PR review team will resume merging on the morning of the keynote address for the regularly scheduled daily publish cycles. You don't need to take further action on this PR; we'll merge it on the day of the keynote address.

Don't add more commits to this reviewed PR.

Thanks.