fix typo & wording in mig demo

mig/README.md

@@ -1,5 +1,5 @@
-This lab is used to get family with MIG (Managed Instance Group). 
-Container is easier to use. 
+This lab is used to understand with MIG (Managed Instance Group). 
+By deploying containers on Compute Engine, you can simplify app deployment while controlling your VM infrastructure, so we will use Container-Optimized OS (COS) image with Docker installed and launches your container when your VM starts up
 
 * https://cloud.google.com/compute/docs/instance-groups/creating-groups-of-managed-instances
 * https://cloud.google.com/compute/docs/instance-groups/rolling-out-updates-to-managed-instance-groups