Update help file for "Get-AzureRMWebAppMetrics" cmdlet to use correct syntax in example

[femid]

Example uses "Get-AzureRmAppServicePlanMetrics" instead of "Get-AzureRmWebAppMetrics"

Description

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 (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[adxsdkps]

Can one of the admins verify this patch?

[maddieclayton]

@azuresdkci Add to whitelist

[maddieclayton]

Closing in favor of #8544 - this is open against our currently active "master" branch.