bpf, docs: fix BPF_NEG entry in instruction-set.rst

This patch fixes the documentation of the BPF_NEG instruction to
denote that it does not use the source register operand.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Acked-by: Dave Thaler <dthaler@microsoft.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230726092543.6362-1-jose.marchesi@oracle.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>