Improve the neural network lecture in ML ND

[kinshuk4]

https://classroom.udacity.com/nanodegrees/nd013/parts/fbf77062-5703-404e-b60c-95b78b2f3f9e/modules/6df7ae49-c61c-4bb2-a23e-6527e69209ec/lessons/83a4e710-a69e-4ce9-9af9-939307c0711b/concepts/e19213bb-bf3e-4d16-aa8d-fe80d5e008f1