Update of New-AzureRmAnalysisServicesFirewallConfig.md - fixed wrong parameter name + a couple of other small content changes

[stiglyt]

Description

Removed "s" from the end of the parameter "FirewallRules".
Changed description in the example so that it includes that PowerBI access is enabled.
Changed releated link so it points to New-AzureRmAnalysisServicesFirewallRule instead of referencing itself.

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