Update concept-reserved-pricing.md

Added Flexible sever

Author: kabharati

articles/postgresql/single-server/concept-reserved-pricing.md

@@ -14,6 +14,7 @@ ms.date: 06/24/2022
 # Prepay for Azure Database for PostgreSQL compute resources with reserved capacity
 
 [!INCLUDE [applies-to-postgresql-single-server](../includes/applies-to-postgresql-single-server.md)]
+[!INCLUDE [applies-to-postgresql-flexible-server](../includes/applies-to-postgresql-flexible-server.md)]
 
 [!INCLUDE [azure-database-for-postgresql-single-server-deprecation](../includes/azure-database-for-postgresql-single-server-deprecation.md)]