Update exercise_solutions.md
Update exercise_solutions.md

Fix to fill missing values with the mean count from the corresponding species as opposed to using the full column mean.
madjelan/scipy2015_tutorial,PepSalehi/scipy2015_tutorial,fonnesbeck/scipy2015_tutorial,ankoorb/scipy2015_tutorial,rouseguy/scipy2015_tutorial,springcoil/scipy2015_tutorial