docs: Fix a few typos
There are small typos in:
- docs/peewee/query_examples.rst
- peewee.py
- tests/model_save.py
- tests/postgres_helpers.py

Fixes:
- Should read `hierarchy` rather than `heirarchy`.
- Should read `existent` rather than `existant`.
- Should read `dictionary` rather than `dictioary`.
- Should read `denormalization` rather than `denormalisation`.



Semi-automated pull request generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md