get_absolute_url uses incorrect url_for arguments.

[adamlwgriffiths]

Post defines the following function

def get_absolute_url(self):
        return url_for('post', kwargs={"slug": self.slug})

This function isn't used in the tutorial, but calling it results in a BuildError exception.
The Blueprint for Post is 'posts' not 'post'.
It also doesn't define which 'function' to call, Ie 'posts.detail' or 'posts.list'.

The correct code should be

def get_absolute_url(self):
        return url_for('posts.detail', slug=self.slug)