backport 44936fd to other language and pdf

This change needs to be backported to all languages. The pdf toolchain
processes this formula with a transformation to some latex mathematic
typesetting and therefore needs to be updated.
Merge branch 'fix_math_typeset'
[it] updated chapter 3
[it] Chapter 2: Reviewed 'Getting a Git Repository'
[it] Reviewed "Cloning an Existing Repository"
[it] Reviewed "Recording Changes to the Repository"