Update Comments in learning.py + PluralityLearner Update (#315)

* Update Comments on learning.py + PluralityLearner

- Fixed some capitalization, spelling and quotation mistakes in comments

- In the PluralityLearner function, the nested function "predict" always returns the same output for a dataset, without taking into account the input ("example"). I defaulted the input as an empty list, so that we don't have to create (or find) a dummy example when we want to simply find the most popular class.

* Update learning.py

* Update learning.py

* Update learning.py

* Made Requested Changes