Update naive_bayes.rst

Updated the Bernoulli equation for P(x_i|y) to match http://www.inf.ed.ac.uk/teaching/courses/inf2b/learnnotes/inf2b-learn-note07-2up.pdf. The equation currently displayed can't possibly be correct because it gives a 0 probability no matter what value x_i takes.
Merge pull request #2858 from ameasure/patch-1

Update naive_bayes.rst