Corrected another typo in aggregation docs. Thanks to Ivan Sagalaev f…

…or the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9752 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/db/aggregation.txt

@@ -39,7 +39,7 @@ used to track the inventory for a series of online bookstores:
        rating = models.FloatField()
        authors = models.ManyToManyField(Author)
        publisher = models.ForeignKey(Publisher)
-       pubdate = models.DateField
+       pubdate = models.DateField()
 
     class Store(models.Model):
        name = models.CharField(max_length=300)