Update building-autoencoders-in-keras.html

[FrancoisPl]

The train and val loss with 10e-5 are 0.14 whereas they are 0.11 with 10e-8 (0.12 with 5e-7). However the new activity mean for th test set is 5.31. I don't know where the discrepancy with the 10e-5 comes from.