Update copyright to pass Lintian test

Builds on Debian fail on Lintian's helper-templates-in-copyright [1]
test.

This mailing list thread [2] explains the issue (and the solution).
The updated copyright file passes the Lintian test.

[1]: https://lintian.debian.org/tags/helper-templates-in-copyright.html
[2]: https://lists.debian.org/debian-mentors/2011/09/msg00450.html