samples/bpf: Reduce the sampling interval in xdp1_user

If interval is 2, and sum - prev[key] = 1, the result = 0. This will
mislead the tester that the port has no traffic right now. So reduce the
sampling interval to 1.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220419114746.291613-1-shaozhengchao@huawei.com