update service availability

Author: duongau

articles/frontdoor/front-door-overview.md

@@ -96,7 +96,7 @@ For a comparison of supported features in Azure Front Door, see [Tier comparison
 
 ## Where is the service available?
 
-Azure Front Door Classic/Standard/Premium SKUs are available in Microsoft Azure (Commercial) and Azure Front Door Classic SKU is available in Microsoft Azure Government (US).
+Azure Front Door Standard, Premium and Classic tiers are available in Microsoft Azure (Commercial) and Microsoft Azure Government (US).
 
 ## Pricing