Fix typo.

Noted by Magnus Hagander.

src/backend/commands/vacuum.c

@@ -894,7 +894,7 @@ vacuum_rel(Oid relid, VacuumStmt *vacstmt, bool do_toast, bool for_wraparound,
 		onerel->rd_rel->relkind != RELKIND_TOASTVALUE)
 	{
 		ereport(WARNING,
-				(errmsg("skipping \"%s\" --- cannot only non-tables or special system tables",
+				(errmsg("skipping \"%s\" --- cannot vacuum non-tables or special system tables",
 						RelationGetRelationName(onerel))));
 		relation_close(onerel, lmode);
 		PopActiveSnapshot();