Update chapter6.md

修改拼写错误fassets/images/figures/signup_mockup_bootstrap.png => assets/images/figures/signup_mockup_bootstrap.png
fixed syntax error
Update chapter6.md

修改错别字 转本=>专门
Update chapter6.md

修改错别字
Update chapter6.md

修改错别字
Update chapter6.md

修改错别字
Merge pull request #6 from NaixSpirit/gh-pages

修改第六章中一个链连拼写错误