bpf, docs: Fix definition of BPF_NEG operation

Instruction is an arithmetic negative, not a bitwise inverse.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230627213912.951-1-dthaler1968@googlemail.com