bpftool: Fix typo in usage help

The usage help for "bpftool prog help" contains a ° instead of the _
symbol for cgroup/sendmsg_unix. Fix the typo.

Fixes: 8b3cba98

 ("bpftool: Add support for cgroup unix socket address hooks")
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20240717134508.77488-1-donald.hunter@gmail.com