Lab 5.2 suggested corrections

In Lab 5.2, you fetch "coreos.com/etcd:v3.1.7." You also fetch "docker://nginx". Then, you list the existing images and show three images including "coreos.com/etcd:v3.1.1." (Note v3.1.1) We never fetched "coreos.com/etcd:v3.1.1." So, when we try to execute the command:

student@ubuntu:~$ sudo rkt image rm coreos.com/etcd:v3.1.1

it will fail. We can execute the command:

student@ubuntu:~$ sudo rkt image rm coreos.com/etcd:v3.1.7