Switch license to SPDX identifier

PKGBUILD

-# Maintainer: Florian Pritz <flo@xinu.at>
+# Maintainer: Robin Candau <antiz@archlinux.org>
+# Contributor: Florian Pritz <flo@xinu.at>
 # Contributor: Jan Fader <jan.fader@web.de>
+
 pkgbase=highlight
 pkgname=(highlight highlight-gui)
 pkgver=4.15
 pkgrel=2
 url="http://www.andre-simon.de/doku/highlight/highlight.html"
-license=('GPL')
+license=('GPL-3.0-or-later')
 arch=('x86_64')
 makedepends=(qt5-base lua boost)
 source=(http://www.andre-simon.de/zip/$pkgname-$pkgver.tar.bz2{,.asc}