debian/copyright: Use https URL for format

According to Debian policy's upgrading checklist, the https form of
the copyright-format URL is now allowed and preferred in the Format
field.
diff --git a/debian/copyright b/debian/copyright
index aac5b7a..d4466ce 100644
--- a/debian/copyright
+++ b/debian/copyright
@@ -1,4 +1,4 @@
-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: XZ Utils
 Upstream-Contact:
  Lasse Collin <lasse.collin@tukaani.org>