no params array available - use imgDir directly

If you call the grid function in a template with additional parameters these parameters are injected directly into the block, see https://github.com/Abhoryo/APYDataGridBundle/blob/master/Twig/DataGridExtension.php?source=cc#L313

`return $template->renderBlock($name, array_merge($this->environment->getGlobals(), $parameters, $this->params));`

So there won't be a params array as shown in the example.