[1.6.x] Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.
Backport of 7442eb1a24 from master.

docs/topics/forms/modelforms.txt

@@ -162,6 +162,7 @@ Consider this set of models::
         title = models.CharField(max_length=3, choices=TITLE_CHOICES)
         birth_date = models.DateField(blank=True, null=True)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.name