doc: arch: correct links in x86.rst

Correctly reference uefi/uefi.rst and uefi/u-boot_on_efi.rst.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>