Fix ThemeHandlerInterface::setDefault() deprecation code example in header comment

The header documentation is wrong. The original issue #2635784: Deprecate ThemeHandler::setDefault() in favour of configuration and replace usages fixed this in all the tests, and initially commented with $this->config() assuming that TestBase was the class. In comment 40 this was suggested to be \Drupal::config() and that was changed in the comment. The problem is that \Drupal::config() returns the readonly config object whereas TestBase::config() returns the editable config object. We need the editable config object. Attached is a patch that fixes this comment.