bpf: doc: "neg" opcode has no operands

Fixes a copy-paste-o in the BPF opcode table: "neg" takes no arguments
and thus has no addressing modes.

Signed-off-by: Dave Anderson <danderson@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>