For some reason Arch and Antergos do not have this simple command that many other distros have. There may be a technical reason, I just don’t know what.

But here’s the contents of that simple script:

#!/bin/sh
grub-mkconfig -o /boot/grub/grub.cfg

and you can put it to your system, e.g. with these commands:

su
cd /usr/local/bin
echo '#!/bin/sh' > update-grub
echo 'grub-mkconfig -o /boot/grub/grub.cfg' >> update-grub
chmod +x update-grub

If some day Antergos officially releases this command, then you need to remove this by:

sudo rm /usr/local/bin/update-grub

before installing the official command.