Update allowselfservicepurchase-powershell.md

[msbemba]

trafficstars

Fixed the example with foreach

Fixes https://github.com/MicrosoftDocs/microsoft-365-docs-pr/issues/23553

[msbemba]

@JohanFreelancer9 Please copy review

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

Learn Build status updates of commit 122957c:

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[yogkumgit]

@cmcatee-MSFT Please merge this PR. Thanks