update submodule grub to public one

.gitmodules

 [submodule "grub/bootiful-grub"]
 	path = grub/bootiful-grub
-	url = ssh://git@ssh.hesge.ch:10572/dimitri.lizzi/bootiful-grub.git
+	url = https://gitedu.hesge.ch/dimitri.lizzi/bootiful-grub.git