samples/bpf: Fix double word in comments

Remove the repeated word "by" in comments.

Signed-off-by: Shaomin Deng <dengshaomin@cdjrlc.com>
Link: https://lore.kernel.org/r/20221017142303.8299-1-dengshaomin@cdjrlc.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>