(GH-168) Fix v3 install code

[michaeltlombardi]

PR Summary

The v3 preview of the PSDesiredStateConfiguration module was unlisted from the PowerShell Gallery. This change:

• updates the installation code snippet to work with the unlisted module
• fixes #168

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 b4fb6ca:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
dsc/docs-conceptual/dsc-3.0/overview.md	:white_check_mark:Succeeded	View (dsc-3.0)

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