Update installation.rst

Adding notice that the AcmeDemoBundle is only accessible in the dev environment. So that newbies are less likely to see an Exception right from the start.
afaik this is not necessary, because console commands are normaly called in the dev environment.. so der server 'created' by a console command also runs in the dev environment..

if you try to run the command with env=prod you run in the same issue as if you dont at the app_dev on apache

.. so its only necessary to add app_dev.php if you run it on apache