Update powershell-get-custom-domain-replace-cert.md

[CBrasga]

Using a filter on the Get-MgBetaApplication command is more efficient than getting all applications each time then filtering via Where-Object.

[prmerger-automator]

@CBrasga : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 5cdd619:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/identity/app-proxy/scripts/powershell-get-custom-domain-replace-cert.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[Court72]

@kenwith

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team