Update nodeapi_example.module for CVS HEAD

Attached patch updates contributions/docs/developer/examples/nodeapi_example.module for CVS HEAD. Please review, especially the code that determines the content-type:

  if ($form_id == 'node_type_form') {
    $type = $form['old_type']['#value'];
  } else {
    $type = $form['type']['#value'];
  }

This change relates to system-defined content-types not having [type][#value] as described in http://drupal.org/node/80891