Update comment in header of errcodes.txt

This file mentions all the files generated from it, but missed that
errcodes-list.sgml is no more, while errcodes-table.sgml is.

Author: Noriyoshi Shinoda
Discussion: https://postgr.es/m/TU4PR8401MB0430855D6B971E49EB55F328EE3E0@TU4PR8401MB0430.NAMPRD84.PROD.OUTLOOK.COM

src/backend/utils/errcodes.txt

@@ -18,7 +18,7 @@
 #   src/pl/tcl/pltclerrcodes.h
 #      the same, for PL/Tcl
 #
-#   doc/src/sgml/errcodes-list.sgml
+#   doc/src/sgml/errcodes-table.sgml
 #      a SGML table of error codes for inclusion in the documentation
 #
 # The format of this file is one error code per line, with the following