(fix) incorrect name completion for /u mail sendto

Author: Lori3f6

src/main/java/cat/nyaa/ukit/mail/MailFunction.java

@@ -309,7 +309,9 @@ public List<String> tabComplete(CommandSender sender, Command command, String al
                     return null;
                 }
             } else if (args[0].equalsIgnoreCase("sendto")) {
-                return Bukkit.getOnlinePlayers().stream().map(Player::getName).toList();
+                if (args.length == 2)
+                    return Bukkit.getOnlinePlayers().stream().map(Player::getName).filter(t -> t.toLowerCase().startsWith(args[1].toLowerCase())).toList();
+                else return null;
             } else {
                 return null;
             }