docs: Fix ref to skiboot-6.4 in 6.5 release notes
docs: Fix ref to skiboot-6.4 in 6.5 release notes

I like to click things.

Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

open-power/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot