Bug#970863: nagstamon: Correct GPL-2 in copyright to GPL-2+

[Bastian Germann]

Source: nagstamon
Severity: normal

Please correct the copyright file to say GPL-2+ (which is the package's
license) instead of GPL-2 (only). This has an impact on the package
because it is a derivative work of pyqt5, which is GPL-3 licensed and
thus incompatible with GPL-2 only.