Replace deprecated PrimaryKeyField with AutoField in docs
Danke.
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<summary>Grep after change</summary>

    > rg -i 'PrimaryKeyField'
    CHANGELOG.md
    885:Re-add a shim for `PrimaryKeyField` (renamed to `AutoField`) and log a
    
    tests/fields.py
    450:class TestCompositePrimaryKeyField(ModelTestCase):
    
    peewee.py
    124:    'PrimaryKeyField',  # XXX: Deprecated, change to AutoField.
    4346:class PrimaryKeyField(AutoField):
    4348:        __deprecated__('"PrimaryKeyField" has been renamed to "AutoField". '
    4351:        super(PrimaryKeyField, self).__init__(*args, **kwargs)
    
    docs/peewee/changes.rst
    56:* :py:class:`PrimaryKeyField` has been renamed to :py:class:`AutoField`

</details>