libbpf: add perf_buffer_ prefix to README

perf_buffer "object" is part of libbpf API now, add it to the list of
libbpf function prefixes.

Suggested-by: Daniel Borkman <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>