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.
good point! I think we could even make this a box `..note::` to be more visible. and say that the AcmeDemoBundle is what you see on the homepage, or something like this. for the noob, it is not obvious what we are saying without a bit of further explanation.