Update copyright years

Signed-off-by: Baiju Muthukadan <[email protected]>

Author: baijum

copyright.tex

@@ -5,9 +5,9 @@
 {\sf First Edition}\\
 \textit{(English)}\\[3mm]
 {\sf\bfseries Baiju Muthukadan}\\[.1in]
-{\small \copyright}{\small \tt 2023} {\small Baiju Muthukadan <[email protected]>}\\
+{\small \copyright}{\small \tt 2015-2023} {\small Baiju Muthukadan <[email protected]>}\\
 
-Essentials of Go Programming {\small \copyright} 2023 by Baiju Muthukadan is licensed
+Essentials of Go Programming {\small \copyright} 2015-2023 by Baiju Muthukadan is licensed
 under Attribution-ShareAlike 4.0 International. To view a copy of this license,
 visit:\\
 \url{http://creativecommons.org/licenses/by-sa/4.0/}