Updated docstring to correctly reflect where 'now' is coming from.

[abenezerBelachew]

Updated docstring to correctly reflect where now is coming from in the AutoCreatedField and AutoLastModifiedField field models.

Problem

Docstring was showing that the now function was coming from datetime instead of django.utils.timezone

Solution

Replaced datetime.now to django.utils.timezone.now in the docstrings

Commandments

• Write PEP8 compliant code.
• Cover it with tests (Doesn't break any tests)
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).