Update tutorial-simple-php-mysql-app.md (Typo "flexible server"→"Flex…

[hyoshioka0128]

…ible Server")

https://github.com/MicrosoftDocs/azure-databases-docs/blob/main/articles/mysql/flexible-server/tutorial-simple-php-mysql-app.md
#PingMSFTDocs

[prmerger-automator]

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

[hyoshioka0128]

@shreyaaithal - related #9

[learn-build-service-prod]

Learn Build status updates of commit 859bb4f:

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[Court72]

Thanks, @hyoshioka0128!