install: Add -U to genfstab examples

[wezm]

I think using UUIDs in fstab is generally preferred, and the example later on uses them as well:

An example /etc/fstab for a root partition and ESP may look like this:

UUID=... / ext4 defaults 0 1
UUID=... /boot/efi vfat defaults 0 2

With this change the generated fstab will better match the example.