Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/ref/contrib/gis/layermapping.txt

@@ -60,6 +60,8 @@ Example
         name = models.CharField(max_length=25) # corresponds to the 'str' field
         poly = models.PolygonField(srid=4269) # we want our model in a different SRID
         objects = models.GeoManager()
+
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return 'Name: %s' % self.name