Fix function name in error hint

pg_read_file() is the function that's in core, pg_file_read() is in
adminpack. But when using pg_file_read() in adminpack it calls the *C*
level function pg_read_file() in core, which probably threw the original
author off. But the error hint should be about the SQL function.

Reported-By: Sergei Kornilov
Backpatch-through: 11
Discussion: https://postgr.es/m/373021616060475@mail.yandex.ru

src/backend/utils/adt/genfile.c

@@ -250,7 +250,7 @@ pg_read_file(PG_FUNCTION_ARGS)
 				 (errmsg("must be superuser to read files with adminpack 1.0"),
 		/* translator: %s is a SQL function name */
 				  errhint("Consider using %s, which is part of core, instead.",
-						  "pg_file_read()"))));
+						  "pg_read_file()"))));
 
 	/* handle optional arguments */
 	if (PG_NARGS() >= 3)