selftest/bpf: Fix rst formatting in readme

A couple of places in the readme had invalid rst formatting causing the
rendering to be off. This patch fixes them with minimal edits.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201122022205.57229-2-andreimatei1@gmail.com