Fix erroneous code reference in tutorial

[sotlampr]

From:

from djangotoolbox.fields import EmbeddedModelField
    tags = ListField()
    comments = ListField(EmbeddedModelField('Comment')) # <---


class Comment(models.Model):
    created_on = models.DateTimeField(auto_now_add=True)
    [...]

to:

from djangotoolbox.fields import EmbeddedModelField

class Comment(models.Model):
    created_on = models.DateTimeField(auto_now_add=True)
    [...]