Make ovn_migration.sh generate-inventory reference the right next step

Otherwise the admin could skip a step.

Change-Id: I9cd6031f88c064dba457d033aa072b6ed918840d
Closes-Bug: 1814826

migration/tripleo_environment/ovn_migration.sh

@@ -199,7 +199,7 @@ EOF
     cat hosts_for_migration
     echo "***************************************"
     echo "Generated the inventory file - hosts_for_migration"
-    echo "Please review the file before running the next command - reduce-mtu"
+    echo "Please review the file before running the next command - setup-mtu-t1"
 }
 
 # Check if the public network exists, and if it has floating ips available