bpf, docs: Fix typo "BFP_ALU" to "BPF_ALU"

"BFP" should be "BPF"

Signed-off-by: Kosuke Fujimoto <fujimotokosuke0@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220609083937.245749-1-fujimotoksouke0@gmail.com