MANUAL: fix copyright years

Author: tarleb

MANUAL.txt

@@ -7542,7 +7542,7 @@ For full documentation, see the [pandoc-lua] man page.
 
 # Authors
 
-Copyright 2006--2022 John MacFarlane ([email protected]). Released
+Copyright 2006--2024 John MacFarlane ([email protected]). Released
 under the [GPL], version 2 or greater.  This software carries no
 warranty of any kind.  (See COPYRIGHT for full copyright and
 warranty notices.) For a full list of contributors, see the file