Fixes #11853 - Add exec to built-in functions doc

[sdwheeler]

PR Summary

Add exec to built-in functions doc

• Fixes AB#400617
• Fixes Add missing function to about_Built-in_Functions #11853

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 afd0ffa:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/7.4/Microsoft.PowerShell.Core/About/about_Built-in_Functions.md	✅Succeeded	View (powershell-7.4)
reference/7.5/Microsoft.PowerShell.Core/About/about_Built-in_Functions.md	✅Succeeded	View (powershell-7.5)
reference/7.6/Microsoft.PowerShell.Core/About/about_Built-in_Functions.md	✅Succeeded	View (powershell-7.6)

For more details, please refer to the build report.

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