Fixed #20159 -- Mispelled attribute in multi-db documentation example.

Thanks to sane4ka.sh at gmail for the report!

docs/topics/db/multi-db.txt

@@ -324,7 +324,7 @@ from::
             in the master/slave pool.
             """
             db_list = ('master', 'slave1', 'slave2')
-            if obj1.state.db in db_list and obj2.state.db in db_list:
+            if obj1._state.db in db_list and obj2._state.db in db_list:
                 return True
             return None