Incorrect documentation about hook_field_formatter_info_alter

API page: https://api.drupal.org/api/drupal/core%21modules%21field%21field.api.php...

As seen in this comment as well https://api.drupal.org/comment/57233#comment-57233 (yes, I commented first, sorry) the code sample is wrong in the documentation.
The 'field types' key is actually 'field_types' so the correct code would be:

function hook_field_formatter_info_alter(array &$info) {
  // Let a new field type re-use an existing formatter.
  $info['text_default']['field_types'][] = 'my_field_type';
}