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

Commit c34a06c56df7 ("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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/Documentation/bpftool-map.rst

···
 |		| **lru_percpu_hash** | **lpm_trie** | **array_of_maps** | **hash_of_maps**
 |		| **devmap** | **devmap_hash** | **sockmap** | **cpumap** | **xskmap** | **sockhash**
 |		| **cgroup_storage** | **reuseport_sockarray** | **percpu_cgroup_storage**
-|		| **queue** | **stack** | **sk_storage** | **struct_ops** }
+|		| **queue** | **stack** | **sk_storage** | **struct_ops** | **ringbuf** }
 
 DESCRIPTION
 ===========

tools/bpf/bpftool/map.c

···
 		"                 lru_percpu_hash | lpm_trie | array_of_maps | hash_of_maps |\n"
 		"                 devmap | devmap_hash | sockmap | cpumap | xskmap | sockhash |\n"
 		"                 cgroup_storage | reuseport_sockarray | percpu_cgroup_storage |\n"
-		"                 queue | stack | sk_storage | struct_ops }\n"
+		"                 queue | stack | sk_storage | struct_ops | ringbuf }\n"
 		"       " HELP_SPEC_OPTIONS "\n"
 		"",
 		bin_name, argv[-2]);