Use secure copyright file specification URI.

Changes-By: lintian-brush
Fixes: lintian: insecure-copyright-format-uri
See-also: https://lintian.debian.org/tags/insecure-copyright-format-uri.html

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: float-debug-proxy
 Source: <https://git.autistici.org/ai3/tools/float-debug-proxy>