docs/system/ppc/ppce500: Use qemu-system-ppc64 across the board(s)

docs/system/ppc/ppce500: Use qemu-system-ppc64 across the board(s)

The documentation suggests that there is a qemu-system-ppc32 binary
while the 32 bit version is actually just named qemu-system-ppc. Settle
on qemu-system-ppc64 which also works for 32 bit machines and causes
less clutter in the documentation.

Found-by: BALATON Zoltan <balaton@eik.bme.hu>
Suggested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20221018210146.193159-2-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>