Merge pull request #82 from clark-archer/section-12.1-typos

Fix typos in section 12.1.
Merge pull request #83 from clark-archer/section-12.2-typos

Fix some typos in section 12.2.
Add missing '+' in Exa. 13.1.3.
Merge branch 'section-13.1-typos' of https://github.com/clark-archer/mooculus into development
fixed minor typo