"azure vm list" command should be changed to "azure vm image list" in the README.md

In the `readme.md` file, `azure vm list` command should be changed to `azure vm image list`. The former lists all the VMs running in the current Azure subscription and the latter displays list of official available images.

The line is located just below the following line:

`To get a list of official available images, run:`

Closes #5.