Update hashtable documentation with cloning note

[OscarFalck17]

Added note about cloning hashtables with a single key and provided an example using PowerShell.

PR Summary

This changes fixes problem in documentation containg how to clone hashtable keys to enumerate over them when the hashtable contains a single key.

[learn-build-service-prod]

PoliCheck Scan Report

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[learn-build-service-prod]

Learn Build status updates of commit 2b67416:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/learn/deep-dives/everything-about-hashtable.md	✅Succeeded	View (>=powershell-5.1)

For more details, please refer to the build report.