Update copyright year

Thinko in 40b3e2c2.
Reported-by: "Wang, Shenhao" <wangsh.fnst@cn.fujitsu.com>
Discussion: https://postgr.es/m/ed98706b82694b57a8c0d339a10732aa@G08CNEXMBPEKD06.g08.fujitsu.local

src/backend/catalog/pg_cast.c

@@ -3,7 +3,7 @@
  * pg_cast.c
  *	  routines to support manipulation of the pg_cast relation
  *
- * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  *