board/pc/post-build.sh: fix typo in grub boot.img path

board/pc/post-build.sh: fix typo in grub boot.img path

Commit 3468ef16fa55610bae3bacefaf816231a20cfab0
("configs/pc_x86_64_efi: use genimage GPT partition table support")
had a small typo on the path to grub boot.img file: i387-pc instead of
i386-pc, which causes a build failure.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/378314412

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Alexandre PAYEN <alexandre.payen@smile.fr>
Cc: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>