Small grammar fix: replaced "it's" with "its"
Merge pull request #207 from dtikhonov/master

Small grammar fix: replaced "it's" with "its"
Fixed typo: "walk through," not "walk though."
Merge pull request #208 from dtikhonov/master

Fixed typo: "walk through," not "walk though."
Fix a typo in "contributing" section of chapter 6
Merge pull request #209 from MasoodFallahpoor/ch06-typo-fix

Fix a typo in "contributing" section of chapter 6