libre: pureos-archive-keyring: fix copyright header
    
    The provenance is given by the following commit:
        commit b352336136d45de172ed012d0871f11dc1ecf672
        Author: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
        Date:   Thu Oct 25 00:48:43 2018 +0200
    
            Add pureos-archive-keyring
    
            This PKGBUILD is based on arch's ubuntu-keyring version 2018.02.28-1.0
    
    This reverts commit 0b3346d1338d6ee2c813996897aa53d9c9e641e6.
    (libre: pureos-archive-keyring: Add copyright header).
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>