DOCS-619 Fix class name in flask tutorial

Author: rozza

source/tutorial/write-a-tumblelog-application-with-flask-mongoengine.txt

@@ -819,7 +819,7 @@ class and create new classes for the new post types. Update the
 
 .. code-block:: python
 
-   class Post(db.Dynamic Document):
+   class Post(db.DynamicDocument):
        created_at = db.DateTimeField(default=datetime.datetime.now, required=True)
        title = db.StringField(max_length=255, required=True)
        slug = db.StringField(max_length=255, required=True)