Fixed #6759: Corrected example of get_db_prep_save() in docs/custom_model_fields.txt

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7301 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/custom_model_fields.txt

@@ -401,8 +401,8 @@ For example::
         # ...
 
         def get_db_prep_save(self, value):
-            return ''.join([''.join(l) for l in (self.north,
-                    self.east, self.south, self.west)])
+            return ''.join([''.join(l) for l in (value.north,
+                    value.east, value.south, value.west)])
 
 ``pre_save(self, model_instance, add)``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~