Update index.yml

Author: msmbaldwin

articles/key-vault/secrets/index.yml

@@ -7,7 +7,7 @@ metadata:
   title: Secrets 
   description: Azure Key Vault enables Microsoft Azure applications and users to store secrets, such as passwords, database connection strings, or keys of an Azure Storage account.
   ms.service: key-vault
-  ms.subservice: certificates
+  ms.subservice: secrets
   ms.topic: landing-page
   author: msmbaldwin
   ms.author: mbaldwin