Fix error message typo.

Reported-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAEoWx2mMmm9fTZYgE-r_T-KPTFR1rKO029QV-S-6n=7US_9EMA@mail.gmail.com

src/backend/utils/adt/formatting.c

@@ -1825,7 +1825,7 @@ str_casefold(const char *buff, size_t nbytes, Oid collid)
 		ereport(ERROR,
 				(errcode(ERRCODE_INDETERMINATE_COLLATION),
 				 errmsg("could not determine which collation to use for %s function",
-						"lower()"),
+						"casefold()"),
 				 errhint("Use the COLLATE clause to set the collation explicitly.")));
 	}