Update perf.nb

speed up pisum(about 7 times faster)
I believe that changes is a reasonable, just direct translation from other languages. For example:
function sum = pisum(ignore)
    sum = 0.0;
    for j=1:500
        sum = 0.0;
        for k=1:10000
            sum = sum + 1.0/(k*k);
        end
    end
end
https://github.com/JuliaLang/julia/blob/master/test/perf/micro/perf.m