Update Rules.php

Finally passing correct DBGroup to Database::connect; codeigniter4#1326

system/Validation/Rules.php

@@ -157,7 +157,7 @@ public function is_unique(string $str = null, string $field, array $data): bool
 		// Break the table and field apart
 		sscanf($field, '%[^.].%[^.]', $table, $field);
 
-		$db = Database::connect();
+		$db = Database::connect($data['DBGroup'] ?? null);
 
 		$row = $db->table($table)
 				  ->select('1')