docs/bpf: Remove source code links

Make path to bench_ringbufs.c just a text, not a special link.

Fixes: 97abb2b3

 ("docs/bpf: Add BPF ring buffer design notes")
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200915005031.2748397-1-andriin@fb.com