Exploding and Vanishing Gradients Derivation

While trying to calculate the maximum value of ds_j/d_s_(j-1), sir has written sigma_prime(aj) = aj(1-aj); if I am write, shouldn’t it be sigma(aj)(1-sigma(aj)) instead? Could you confirm if it indeed is an error? If yes, could you please share the corrected derivation? Thank you so much for your help!

Edit : There won’t be any change in what the results are, only the explanations are incorrect. Could you please correct this?