d/copyright: switch license to AGPL-3+
d/copyright: switch license to AGPL-3+

Fix the license to AGPL-3+ rather than AGPL-3.

Git-Dch: Ignore
Repored-by: Andrey Rahmatullin <a3b3b66f52fbce705a20809c3e7e1435d0ddc7a1@debian.org>
Signed-off-by: Arturo Borrero Gonzalez <6536d6b0723dedf12377d931b2edd2f9488a948b@gmail.com>

aborrero/pkg-liquidprompt