Only show the USB stick in trophy when it's installed

wiki/src/install/inc/steps/create_persistence.inline.mdwn

 <div class="trophy">
 
-<div class="state-image">[[!img install/inc/infography/tails-usb.png link="no" alt="Tails USB stick"]]</div>
-
 <p>Yay, you managed to start Tails on your computer!</p>
 
 <p>If you want to save some of your documents