Updated tutorial INSTALLED_APPS section (removed contrib.sites)

* :mod:`django.contrib.sessions` -- A session framework.

* :mod:`django.contrib.sites` -- A framework for managing multiple sites

  with one Django installation.

* :mod:`django.contrib.messages` -- A messaging framework.

* :mod:`django.contrib.staticfiles` -- A framework for managing

These applications are included by default as a convenience for the common case.

Each of these applications makes use of at least one database table, though,

Some of these applications makes use of at least one database table, though,

so we need to create the tables in the database before we can use them. To do

that, run the following command: