Wrong command used in documentation example

[chayanbaner]

Description

The command used in example was "Add-AzureRmApplicationGatewayCustomError" instead of "Add-AzureRmApplicationGatewayHttpListenerCustomError"

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?

[cormacpayne]

@azuresdkci add to whitelist