[PATCH]: Update URL to HTTPS in debian/copyright

[Kirill Rekhov <krekhov.dev@gmail.com>]

Dear Maintainer(s), sorry for wasting your time.

I was trying to find something wrong in sid-xterm/xterm-393/debian/*

and came across a message from the `duck` utility:

I: debian/copyright:5: URL: http://invisible-island.net/xterm/xterm.html: INFORMATION (Certainty:possible)
   URL schema changed from HTTP to HTTPS during redirect(s): http://invisible-island.net -> https://invisible-island.net
   Please investigate and update the URL eventually, to avoid unnecessary redirects!

I have attached a patch `avoid-unnecessary-redirect.patch` that fixes this
(improving security and avoiding unnecessary redirects), you can use it, thanks.

I also saw that a new version of xterm is available: https://invisible-mirror.net/archives/xterm/xterm-394.tgz
downloaded it, applied patches from version 393, built the package and tested it. Works well!

---
Kirill Rekhov

diff --color -ruN orig/xterm-393/debian/copyright xterm-393/debian/copyright
--- orig/xterm-393/debian/copyright
+++ xterm-393/debian/copyright
@@ -2,7 +2,7 @@
 <ender@debian.org> on Tue, 27 Sep 2005 01:45:34 +0200.
 
 
-Upstream sources: http://invisible-island.net/xterm/xterm.html
+Upstream sources: https://invisible-island.net/xterm/xterm.html
 
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