Typo in Chapter 5

[Jiyuan-Yang]

Hello Dr. David Maxwell and Dr. Leif Azzopardi! In Chapter 5, section Telling Django about Your Database, you've given us some examples about other database engines supported by Django. In the source MarkDown files at this repo, you used PostgresSQL, but I've found that the official website of this database use PostgreSQL. It seems that there is a redundant s here. I think this is an extraordinary book and I've learned a lot, thank you very much for your efforts!

[Jiyuan-Yang]

And in Chapter 5, section Creating Models, in the sentence:

 Implementing the `__str()__` method in your classes will make debugging so much easier...

Maybe this should be __str__()?