Update sys-dm-exec-distributed-requests-transact-sql.md

[VladDBA]

Hi SQL Server Docs Team,

I've corrected a typo and the case-inconsistency between NULL and Null.

Best regards,

Vlad

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

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[Jak-MS]

@rwestMSFT

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[rwestMSFT]

#sign-off

[rwestMSFT]

Thanks again, @VladDBA. We appreciate your close attention.

[prmerger-automator]

PR 9134 has been merged from VladDBA:patch-1 to MicrosoftDocs:live by Court72.

@VladDBA, @rwest, @rwestMSFT