fix error in Building your Deep Neural Network - Step by Step IN [18]

[Freyr-Wings]

the original code wrongly use sigmoid twice and is weird

[pangahn]

error IN [18] please fix it as following dAL = - (np.divide(Y, AL) - np.divide(1 - Y, 1 - AL))

grads["dA" + str(L-1)], grads["dW" + str(L)], grads["db" + str(L)] = linear_activation_backward(dAL, current_cache, "sigmoid")