pdfpc: Update license to match upstream

Upstream changed the license - see
https://github.com/pdfpc/pdfpc/blob/master/CHANGELOG.rst#version-450.

Changed files
+1 -1
pkgs
applications
misc
pdfpc
+1 -1
pkgs/applications/misc/pdfpc/default.nix
···
meta = with lib; {
description = "A presenter console with multi-monitor support for PDF files";
homepage = "https://pdfpc.github.io/";
-
license = licenses.gpl2Plus;
+
license = licenses.gpl3Plus;
maintainers = with maintainers; [ pSub ];
platforms = platforms.unix;
};