Fixed typos in Chapter 1

 * stastical -> statistical
 * modelling -> modeling
Fix typos in Chapter 4

 * poisson -> Poisson
 * probablities -> probabilities
 * distibution -> distribution
 * truely -> truly
 * concious -> conscious
 * Unbeknowst -> Unbeknownst
 * enourmous -> enormous
 * equivilants -> equivalents
 * collegues -> colleagues
 * phenonmenon -> phenomenon
 * substracting -> subtracting
 * implicity -> implicitly
 * effectivy -> effectively
 * exampes -> examples
 * equiviliance -> equivalence
Merge pull request #61 from pablooliveira/Typo-Fixes

Typo fixes in Chapters 1 and 4