Removed usage of to-be-deprecated ADMIN_FOR setting in contributing docs

  .. code-block:: rst

    .. setting:: ADMIN_FOR

    .. setting:: ADMINS

    ADMIN_FOR

    ---------

    ADMINS

    ------

    Default: ``()`` (Empty tuple)

    Used for admin-site settings modules, this should be a tuple of

    settings modules (in the format ``'foo.bar.baz'``) for which this site

    is an admin.

    A tuple that lists people who get code error notifications. When

    ``DEBUG=False`` and a view raises an exception, Django will email these people

    with the full exception information. Each member of the tuple should be a tuple

    of (Full name, email address). Example::

    The admin site uses this in its automatically-introspected

    documentation of models, views and template tags.

        (('John', 'john@example.com'), ('Mary', 'mary@example.com'))

    Note that Django will email *all* of these people whenever an error happens.

    See :doc:`/howto/error-reporting` for more information.

  This marks up the following header as the "canonical" target for the

  setting ``ADMIN_FOR`` This means any time I talk about ``ADMIN_FOR``,

  I can reference it using ``:setting:`ADMIN_FOR```.

  setting ``ADMINS``. This means any time I talk about ``ADMINS``,

  I can reference it using ``:setting:`ADMINS```.

That's basically how everything fits together.

  __ http://sphinx-doc.org/markup/desc.html#info-field-lists

* Whenever possible, use links. So, use ``:setting:`ADMIN_FOR``` instead

  of ````ADMIN_FOR````.

* Whenever possible, use links. So, use ``:setting:`ADMINS``` instead

  of ````ADMINS````.

* Use directives where appropriate. Some directives

  (e.g. ``.. setting::``) are prefix-style directives; they go *before*