Add estimated time and refine exercise instructions

Added estimated time for the exercise and clarified task instructions.

Author: staleycyn

Instructions/Exercises/M06-Unit 7 Deploy and configure Azure Firewall using the Azure portal.md

@@ -19,9 +19,11 @@ Being part of the Network Security team at Contoso, your next task is to create
 
 ![Diagram of virtual network with Azure Firewall architecture.](../media/7-exercise-deploy-configure-azure-firewall-using-azure-portal.png)
 
+## Estimated time: 60 minutes
+
 ## Job skills
 
-In this exercise, you will:
+In this exercise, you:
 
 + Task 1: Create a resource group
 + Task 2: Create a virtual network and subnets
@@ -34,8 +36,6 @@ In this exercise, you will:
 + Task 9: Change the primary and secondary DNS address for the server's network interface
 + Task 10: Test the firewall
 
-## Estimated time: 60 minutes
-
 ## Task 1: Create a resource group
 
 In this task, you will create a new resource group.