main/netcat-openbsd: correct license metadata

As per [0] at the upstream and the source itself in [1], the license is BSD-3-Clause, not GPL. Interestingly, it is one of the alternative variants discussed in [2].

• [0] https://metadata.ftp-master.debian.org/changelogs/main/n/netcat-openbsd/netcat-openbsd_1.217-2_copyright
• [1] http://cdn-fastly.deb.debian.org/debian/pool/main/n/netcat-openbsd/netcat-openbsd_1.130.orig.tar.gz
• [2] https://github.com/spdx/license-list-XML/issues/399