Minor Review: notebooks/machine-learning-fundamentals/exercise-evaluation-metrics.ipynb

• Exercise on kappa score: Renamed p_0 to p_o in the explanation - o as in observed agreement.
• Added a skip cell with an alternate introduction text to the Kappa score to introduce the terms observed agreement and expected agreement. I would appreciate if the next review would please review if any of the additions are useful and should be added to the explanation.
• The exercise on kappa score provided an explanation, but no exercise. Added an exercise to compute the kappa score + a test for verification