MDL-60443 dataformat lib: Change a wrong variable name

The variable $type really is $dataformat. I have changed it for
prevent the error and show a correct error message.

lib/dataformatlib.php

@@ -44,7 +44,7 @@ function download_as_dataformat($filename, $dataformat, $columns, $iterator, $ca
 
     $classname = 'dataformat_' . $dataformat . '\writer';
     if (!class_exists($classname)) {
-        throw new coding_exception("Unable to locate dataformat/$type/classes/writer.php");
+        throw new coding_exception("Unable to locate dataformat/$dataformat/classes/writer.php");
     }
     $format = new $classname;