docs: efi-stub: Fix paths for x86 / arm stubs

This fixes the paths of x86 / arm efi-stub source files.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessos.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220727140539.10021-1-jprvita@endlessos.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>