Fixed name of spec id attr in docs

The ``__init__`` method of ``ImageSpecField`` has no attribute called ``spec_id``. It's called ``id``. Updated docs to reflect.

Code:
https://github.com/matthewwithanm/django-imagekit/blob/develop/imagekit/models/fields/__init__.py#L33

Corresponding doc:
http://django-imagekit.readthedocs.org/en/latest/advanced_usage.html