Few typos, cleanup PC3 SpinBox, mention auroae, explicit code tags for formatting

A complete Window Decoration+Aurorae tutorial can be written based on:

• https://techbase.kde.org/User:Mgraesslin/Aurorae
• The links at https://zren.github.io/kde/repos/#kwin to all the various kdecoration2 plugins
• the text blurb at https://www.reddit.com/r/kde/comments/j3lvdd/what_is_auroraeaurorae_themes/