Fix copyright script
After applying the new copyright script suggested in the Allen copyright checking CI, the full GPL3 license is applied. See eg. https://gitlab.cern.ch/lhcb/Allen/-/merge_requests/414/diffs?diff_id=654470&start_sha=b1cef9293d671b6b0ce5bb68b2e3ef5f5e6f3721
The script should instead, when applied on Allen, only copy the very first line of the license (ie. just the copyright).