bpf, doc: Change right arguments for JIT example code

The example code for the x86_64 JIT uses the wrong arguments when
calling function bar().

Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191114034351.162740-1-maowenan@huawei.com