Fixed callback redirect link

front/callback.php

@@ -52,7 +52,7 @@ if ($signon_provider->login()) {
    }
 
    Html::nullHeader("Login", $CFG_GLPI["root_doc"] . '/index.php');
-   echo '<div class="center spaced"><a href="' + $url_redirect + '">' .
+   echo '<div class="center spaced"><a href="' . $url_redirect . '">' .
    __('Automatic redirection, else click') . '</a>';
    echo '<script type="text/javascript">
          if (window.opener) {