Update chapter06.rst

Lower in this [1] document book said that I can edit user and see changes. Django answer [2] to this:
[2] You cannot add messages without installing django.contrib.messages.middleware.MessageMiddleware
[1] When you edit an existing object, you will notice a History(...).
I'm newbie in Django but, I haven't get uncommented
'django.contrib.messages' in INSTALLED_APPS to get this work.