Fix repetition error

<unique-frontend-app-name> was repeated. Should be <unique-frontend-app-name> and <unique-frontend-app-name>

Author: weineran

articles/app-service/scripts/terraform-secure-backend-frontend.md

@@ -31,7 +31,7 @@ Browse to the [Azure documentation](/azure/developer/terraform/) to learn how to
 
 ## The complete terraform file
 
-To use this file, replace the placeholders _\<unique-frontend-app-name>_ and _\<unique-frontend-app-name>_ (app name is used to form a unique DNS name worldwide). 
+To use this file, replace the placeholders _\<unique-frontend-app-name>_ and _\<unique-backend-app-name>_ (app name is used to form a unique DNS name worldwide). 
 
 ```hcl
 terraform {