redundant 'left' in chapter 4

[micuat]

@genekogan, I found redundant 'left' in this sentence (probably the second one can be omitted):
https://github.com/ml4a/ml4a.github.io/blame/master/_chapters/how_neural_networks_are_trained.md#L169

... and so solving the above linear regression will be left as an exercise left to the reader.

(sorry for pointing out small things, but when I translate I find these things)

[micuat]

and probably 'Recall' in the same paragraph should be succeeded by 'that'

[micuat]

another typo here
https://github.com/ml4a/ml4a.github.io/blame/master/_chapters/how_neural_networks_are_trained.md#L233

'Subsequntly' -> 'Subsequently'

[micuat]

https://github.com/ml4a/ml4a.github.io/blame/master/_chapters/how_neural_networks_are_trained.md#L235

'fluctations' -> 'fluctuations'