Add missing $ in about_Preference_Variables.md

[sethvs]

PR Summary

Add missing $ in about_Preference_Variables.md

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[sdwheeler]

Thanks for the fix

[learn-build-service-prod]

Learn Build status updates of commit 5342317:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Preference_Variables.md	✅Succeeded	View (powershell-5.1)
reference/7.2/Microsoft.PowerShell.Core/About/about_Preference_Variables.md	✅Succeeded	View (powershell-7.2)
reference/7.4/Microsoft.PowerShell.Core/About/about_Preference_Variables.md	✅Succeeded	View (powershell-7.4)
reference/7.5/Microsoft.PowerShell.Core/About/about_Preference_Variables.md	✅Succeeded	View (powershell-7.5)

For more details, please refer to the build report.

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