debian: Add missing '<' in copyright

The email address information should be inside brackets. This
commit ensures the missing bracket character issue is remedied.
Reviewed-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

debian/copyright

@@ -8,7 +8,7 @@ Copyright: 2017-2019 IPFire Development team <info@ipfire.org>
 License: LGPL-2.1
 
 Files: debian/*
-Copyright: 2019 Stefan Schantl stefan.schantl@ipfire.org>
+Copyright: 2019 Stefan Schantl <stefan.schantl@ipfire.org>
 License: LGPL-2.1
 
 License: LGPL-2.1