Readme.md: correct typo

[pelly-ryu]

In https://github.com/ambitioninc/django-activatable-model#basic-usage
and https://github.com/ambitioninc/django-activatable-model#overriding-the-activatable-field-name ,
I think
from activatable_model import BaseActivatableModel
should be
from activatable_model.models import BaseActivatableModel