<?php
if ($modx->event->name == 'OnLoadWebDocument') {
    $uri = $_SERVER['REQUEST_URI'];
    if ($modx->resource->isfolder && substr($uri, -1) != '/' && !strpos($uri, '?')) {
        $modx->sendRedirect($modx->makeUrl($modx->resource->id),array('responseCode' => 'HTTP/1.1 301 Moved Permanently'));
    }
}