Correct the phpdoc license tag of TarLib.class.php

The author of MaxgTar (original name of this file) wrote an incorrect
phpdoc license tag mentionning the “GPL” without a version number.

Contacted to clarify the licensing situation of this file, he explained
that he wanted to use the LGPL-2.1:
> I am the author of MaxgTar and I must say I am more than slightly
> surprised to hear about it.
> […]
> I have no doubt that my intention was to use the GNU Lesser General
> Public License, version 2.1.

inc/TarLib.class.php

@@ -7,7 +7,7 @@
  *
  * Modified for Dokuwiki
  *
- * @license GPL
+ * @license LGPL-2.1
  * @link    http://docs.maxg.info
  * @author  Bouchon <tarlib@bouchon.org> (Maxg)
  * @author  Christopher Smith <chris@jalakai.co.uk>