Fix typo in error template file name.

Fixes #6644

src/Template/Error/missing_controller.ctp

@@ -39,7 +39,7 @@ if (empty($plugin)) {
 $this->layout = 'dev_error';
 
 $this->assign('title', 'Missing Controller');
-$this->assign('templateName', 'missing_controller.php');
+$this->assign('templateName', 'missing_controller.ctp');
 
 $this->start('subheading');
 ?>