Fix linter errors in wikibooks content

[elbotho]

@kulla will organize (or delegate) a sprint to fix the remaining issues as it would solve some problems with the PDF export.

[kulla]

@vroland Is there a statistics output showing all linter issues currently?

[vroland]

@kulla There is, at https://mathe-builds.serlo.org/nightly/Benutzer%40COLON%40Valentin_Roland%40SLASH%40Analysis_1/861270/stats/all/861270.stats.html.