docs/bpf: Fix ringbuf documentation

Remove link to litmus tests that didn't make it to upstream. Fix ringbuf
benchmark link.

I wasn't able to test this with `make htmldocs`, unfortunately, because of
Sphinx dependencies. But bench_ringbufs.c path is certainly correct now.

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/20200910225245.2896991-1-andriin@fb.com