Clarify the definition of i

In line 287 and 295 of your [Neural Nets](https://github.com/frnsys/ai_notes/blob/d92acc4e4f6220d6d9bd0ae777fecfe3c85f5e67/notes/2.03%20Neural%20Nets.md) notes you never actually defined the function i. At least for me it wasn't clear what exactly i should be until I read on.

Maybe it would be better to use the NET^i_j notation there instead of i?