Minor Review: notebooks/differentiable-programming/exercise-activation-functions.ipynb

If everythign is correct, executing the cell below should plot 2 graphs like the following picture:

Not quite - you have to remember to execute the training cell again. Otherwise the gradient log grad_w remains all zeros and the plot is a constant line.

Added a note

With the new implementation in place, execute the training cell again to populate the log of gradients grad_w.

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Minor spelling/typo fixes here and there