fix a bug

I think you mistype <i>a{n + 1} * (1 - a{n})</i> while it actually should be <i>a{n + 1} * (1 - a{n + 1})</i>.

In given parameters settings, <b>hiddenSizeL1</b> is coincidentally equal to <b>hiddenSizeL2</b> so your code can run without dimension incompatible problem, however, if you run <b>checkStackedAECost</b>, it won't compile correctly.

Last but not least, really thanks for your code, very helpful to me.