Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/ref/contrib/admin/actions.txt

@@ -57,6 +57,7 @@ simple news application with an ``Article`` model::
         body = models.TextField()
         status = models.CharField(max_length=1, choices=STATUS_CHOICES)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.title