tools, bpftool: Add ringbuf map type to map command docs

Commit c34a06c5

 ("tools/bpftool: Add ringbuf map to a list of known
map types") added the symbolic "ringbuf" name. Document it in the bpftool
map command docs and usage as well.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200616113303.8123-1-tklauser@distanz.ch