Update how-to-troubleshoot-query-performance.md (Typo "flexible serve…

[hyoshioka0128]

…r"→"Flexible Server")

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

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

[hyoshioka0128]

@SudheeshGH - related #10

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[Court72]

Thanks, @hyoshioka0128!