d-i: (grub) detected existing operating systems should be listed in a bullet form

[Obibok <obibok@gmail.com>]

Hello again.

Currently at the "Install the GRUB boot loader on a hard disk" screen, the 
list of detected operating systems is part of a sentence and all items are 
separated by commas. For a clearer and more professional look, I suggest the 
existing systems be listed in a bullet form.