samples: bpf: Fix syscall_tp openat argument

This modification doesn't change behaviour of the syscall_tp
But such code is often used as a reference so it should be
correct anyway

Signed-off-by: Denys Zagorui <dzagorui@cisco.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231019113521.4103825-1-dzagorui@cisco.com