Fix a typo in section-9.2.1
In 9.20, email address need to transfer lowercase
Merge pull request #67 from zyuanhong/add_sth

Fix a error and typo in chapter 9
Fix typo: ActiveRecored -> ActiveRecord
Fix typo in chapter9
Merge pull request #68 from zyuanhong/add_sth

Fix typo: ActiveRecored -> ActiveRecord
Fix type error in chapter-7
Merge pull request #69 from mengche/patch-1

Fix typo error in chapter-7