Sigmoid Derivative

This proof is to answer @Christian Sanchez question regarding how my code calculates the Sigmoid activation derivative easily:

https://www.mathworks.com/matlabcentral/fileexchange/54076-mlp-neural-network-with-backpropagation?s_tid=mlc_fex_email_activity#feedback-31e1e281-f332-4e40-ba59-5e9c3c8cdc5d