Update New-AzApiManagementSubscription script samples with correct variable names

[pvandenheede]

Description

Some of the script samples contain incorrect variable names.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[ghost]

Thank you for your contribution PVandenheede! We will review the pull request and get back to you soon.

[BethanyZhou]

Thanks for your passions. It's good to unify all examples' variable name.

[BethanyZhou]

/azp run azure-powershell - security-tools

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