debian/copyright: Use HTTPS URL

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>

debian/copyright

-Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
 Upstream-Name: apt-mirror
 Upstream-Contact: Brandon Holtsclaw <me@brandonholtsclaw.com>
 Source: https://github.com/apt-mirror/apt-mirror