Fix incorrect use of pathlib.Path method in README

[pmac]

Path objects have no child() method in the pathlib module of the standard library. The method to use is joinpath(), or the division operator which also joins paths. So it could also be:

    DATABASES = {
        'default': config(
            'DATABASE_URL',
            default='sqlite:///' + (BASE_DIR / 'db.sqlite3'),
            cast=db_url
        )
    }

But that's harder to parse for users unfamiliar with the pathlib module IMO.