Update ServerExternalAdministrator.yml

[myatt83]

fixed typos in "server" (was "sever")

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

Learn Build status updates of commit 79ae776:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docs-ref-autogen/@azure/arm-sql/ServerExternalAdministrator.yml	:white_check_mark:Succeeded	View (azure-node-latest)

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