Add a missed output scentence of section6.2.4 of chapter6
Merge pull request #51 from logchi/add_sth

Add a missed output scentence of section6.2.4 of chapter6
Fix typo on chapter7
Merge pull request #52 from zyuanhong/add_sth

Fix typo on chapter7
Fix error in chapter-7
Merge pull request #53 from zyuanhong/add_sth

Fix error in chapter7
Fix typo in chapter8
Merge pull request #54 from zyuanhong/add_sth

Fix typo in chapter8
Fix error in chapter8
Merge pull request #55 from zyuanhong/add_sth

Fix error in chapter 8
fix link
Merge pull request #56 from lonre/fix-chapter6-link

fix link
Fix typo and indentation in chapter8
Merge pull request #57 from zyuanhong/add_sth

Fix typo and indentation in chapter8