documentation: ciao-down prepare is now ciao-down create

We've changed a bit how ciao-down creates new VMs so it's not tied to
ciao as much as before. See:

  ciao-project/ciao#1362

Update the documentation to follow that change.

Fixes: #1045
Signed-off-by: Damien Lespiau <[email protected]>

Author: Damien Lespiau

documentation/Quickstart-Clear-Containers-in-a-Ubuntu-VM.md

@@ -55,17 +55,17 @@ All you need to do to use ``ciao-down`` on your machine is:
 
 
    ```
-   $ $GOPATH/bin/ciao-down prepare -vmtype clearcontainers
+   $ $GOPATH/bin/ciao-down create clearcontainers
    $ $GOPATH/bin/ciao-down connect
    ```
 
 ## Usage of ``ciao-down``
 
 
 ```
-$ ciao-down [prepare|start|stop|quit|status|connect|delete]
+$ ciao-down [create|start|stop|quit|status|connect|delete]
 
-- prepare : creates a new VM for ciao or clear containers
+- create : creates a new VM
 - start : boots a stopped VM
 - stop : cleanly powers down a running VM
 - quit : quits a running VM