(AB#263705) Fix description for example using `[ValidateScript()]`

[michaeltlombardi]

PR Summary

Prior to this change, the example description in the article How to validate an argument using a script declared that the example validates that the input belongs to a set of possible values. That description belongs to a different example, using [ValidateSet()].

This change:

• Corrects the example description to match the code.
• Fixes AB#263705

PR Checklist

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

[learn-build-service-prod[bot]]

Learn Build status updates of commit 4eed9bc:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/developer/cmdlet/how-to-validate-an-argument-using-script.md	:white_check_mark:Succeeded	View (>=powershell-5.1)

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