Fix about_Regular_Expressions.md

[sethvs]

PR Summary

According to Character Escapes in Regular Expressions
(and even [regex]::escape('3.\d{2,}') example below)
closing square bracket ] and closing squiggle brace } are not special symbols and need not to be escaped.

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.

[learn-build-service-prod]

Learn Build status updates of commit 1b83041:

✅ Validation status: passed

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

For more details, please refer to the build report.

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[sdwheeler]

@sethvs Thanks for the updates.