"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.

Author: dadoonet

README.md

@@ -131,7 +131,7 @@ for more information.
 You will need to choose the operating system you want to run on. To get a list of official available images, run:
 
 ```sh
-azure vm list
+azure vm image list
 ```
 
 Let's say we are going to deploy an Ubuntu image on an extra small instance in West Europe: