You should not modify the grub.cfg-file. This file is auto-generated, and all your changes will be lost the next time an update issues the update-grub command.
If you want to modify Grub's behavior you should modify /etc/default/grub instead. There is an option named GRUB_TIMEOUT. If you set this to zero the time-out should be disabled.
Remember to do
every time you have modified that file, so that Grub can generate a new grub.cfg.