Fix inconsistent variable casing

[revolter]

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[opbld33]

Docs Build status updates of commit da66d87:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/7.1/Microsoft.PowerShell.Core/About/about_Automatic_Variables.md	✅Succeeded	View (powershell-7.1)

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[chasewilson]

Great catch! Thanks for the contribution!