update navigationMenu hook example to show use of 'attr' parameter

docs/hooks/hook_civicrm_navigationMenu.md

@@ -134,7 +134,8 @@ function donortrends_civicrm_navigationMenu(&$params) {
       'separator' => 1,
       'parentID' => $reportID,
       'navID' => $navId,
-      'active' => 1
+      'active' => 1,
+      'attr' => ['target' => '_blank'],   // if you want to open this menu link in a new page, then add '_target' attribute
     ),
   );
 }