Correct GPLv2 license url
Old url points to non-existing page 'gpl-2.1.html'.
Updated url to 'gpl-2.0.html' which matches the license shown in Okular.
Old URL: https://www.gnu.org/licenses/old-licenses/gpl-2.1.html
Updated URL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html