bpf, docs: Fix literal block for example code

Add a missing colon so that the code block followed can be rendered
properly.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210424021208.832116-1-hengqi.chen@gmail.com