Update how-to-manage-firewall-portal.md (Typo "flexible server"→"Flex…

[hyoshioka0128]

…ible Server")

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

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[hyoshioka0128]

@SudheeshGH - related #10