Fix typos in Chapter 3

 * positerior -> posterior
 * ontop -> on top
 * postive -> positive
 * modelling -> modeling
 * determinsitic -> deterministic
 * assingment -> assignment
 * aboves -> above
 * re-reun -> re-run
 * opimization -> optimization
 * attemps -> attempts
 * Tt is -> It is
 * retured -> returned
 * paramter -> parameter
 * esimate -> estimate
 * algoithms -> algorithms
 * specifing -> specifying
Merge pull request #58 from pablooliveira/Chapter3-fixtypos

Fix typos in Chapter 3