Historically, libnm claimed to be LGPL-2.0+ licensed. But as keep mixing that with LGPL-2.1+ code (e.g. from systemd), so probably even the parts that claimed to be LGPL-2.0+, were not entirely correctly doing so. Anyway, since we switched to SPDX license identifiers, we now claim everywhere that the right license is LGPL-2.1+. Update the comment to reflect that.