[docs]fix optimistic locking document in hacks.rst
In peewee3, Model class has no attribute `_dict` , but has `__dict__` .
And in hack.rst, there is no definition of `ConflictDetectedException` . 

Reffered to this blog post. 
http://charlesleifer.com/blog/optimistic-locking-in-peewee-orm/

Thx. 