Make it clear that the user will use PGP

We could want him to *have* one PGP key, but it can be asked after in the overview.

wiki/src/install/debian.mdwn

@@ -36,7 +36,7 @@
     [[
       <div class="panel panel-primary">
         <div class="panel-heading">
-          <h3 class="panel-title"><small>Install from Debian </small><br>using the command line</h3>
+          <h3 class="panel-title"><small>Install from Debian using</small><br>command line <small> and </small> PGP</h3>
         </div>
       </div>
     |expert/usb]]