Revise lab description and add primary topics

Updated the description and primary topics for the lab.

Author: staleycyn

Instructions/Exercises/M03-Unit 4 Configure an ExpressRoute Gateway.md

@@ -1,12 +1,15 @@
----
-lab:
-  title: M03-Unit 4 Configure an ExpressRoute Gateway
-  description: '![Diagram of virtual network gateway.](../media/4-exercise-configure-expressroute-gateway.png)'
-  duration: 60 minutes
-  level: 300
-  islab: true
----
-
+---
+lab:
+  title: M03-Unit 4 Configure an ExpressRoute Gateway
+  description: Configure an ExpressRoute Gateway
+  duration: 60 minutes
+  level: 300
+  islab: true
+  primarytopics:
+  - Azure
+  - ExpressRoute
+---
+
 # M03-Unit 4 Configure an ExpressRoute Gateway
 
 ## Exercise scenario
@@ -122,3 +125,4 @@ Congratulations on completing the lab. Here are the main takeaways for this lab.
 
 
 
+