community/milter-greylist: fix wrong license

It doesn't look like it has been ever licensed under GPL...

http://hcpnet.free.fr/milter-greylist/#license

community/milter-greylist/APKBUILD

@@ -2,11 +2,11 @@
 # Maintainer: Leonardo Arena <rnalrd@alpinelinux.org>
 pkgname=milter-greylist
 pkgver=4.6.2
-pkgrel=1
+pkgrel=2
 pkgdesc="Stand-alone milter written in C that implements the greylist filtering method"
 url="http://hcpnet.free.fr/milter-greylist"
 arch="all"
-license="GPL"
+license="BSD-3-Clause"
 depends=
 depends_dev=
 makedepends="$depends_dev bison flex geoip-dev libmilter-dev libspf2-dev"