Fix error message

Author: kovmir

postgresql_extension_template--1.0.sql

@@ -1,7 +1,7 @@
 /* postgresql_extension_template--1.0.sql */
 
 -- complain if script is sourced in psql, rather than via CREATE EXTENSION
-\echo Use "CREATE EXTENSION pg_show_plans" to load this file. \quit
+\echo Use "CREATE EXTENSION postgresql_extension_template" to load this file. \quit
 
 CREATE FUNCTION my_function()
 RETURNS cstring