Remove-AzureRmWebAppSlot - Fixes inconsistent slot names

[cornem]

Description

Updated the slot name in the sample code to match the name used in the explanation.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• 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:
  • the changes have gone through a cmdlet design review
  • the cmdlet markdown files were generated using the platyPS module

[azuresdkci]

Can one of the admins verify this patch?

[cormacpayne]

@azuresdkci add to whitelist