Corrected error message in Many-to-many relationships docs.

docs/topics/db/examples/many_to_many.txt

@@ -54,7 +54,7 @@ You can't associate it with a ``Publication`` until it's been saved::
     >>> a1.publications.add(p1)
     Traceback (most recent call last):
     ...
-    ValueError: 'Article' instance needs to have a primary key value before a many-to-many relationship can be used.
+    ValueError: "<Article: Django lets you build Web apps easily>" needs to have a value for field "id" before this many-to-many relationship can be used.
 
 Save it!
 ::