fu-util-common: correct the default prompt for reboot/shutdown (Fixes: #1617)
fu-util-common: correct the default prompt for reboot/shutdown (Fixes: #1617)

Y/n means that Y is the default if you press enter.
y/N means that N is the default if you press enter.

fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd