update model estructure in django 3 code example

Example 1: .save() in django

>>> one_entry = Entry.objects.get(pk=1)

Example 2: django id

The ID in django is a "unique identifier" for each models instances.
You do not need to explicitly add a ID field to your own models as 
the ID attribute is automatically generated for you behind the scences
by django for each new model and any newly created instances of that 
model afterwards.