documentation: local_ops fix on_each_cpu

Impact: update code example in documentation

* Dr. David Alan Gilbert (dave@treblig.org) wrote:
[...]
>  I noticed while looking at something else that the example in
>  local_ops.txt still has the 4 operand on_each_cpu in the latest git;
>  I don't know the rest of the code around there very well so I thought
>  it best to mention it rather than post a patch.

Reported-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Ingo Molnar <mingo@elte.hu>