addons section referring to non existent examples/addons folder

[frauzufall]

https://github.com/openframeworks/ofBook/blob/master/chapters/setup_and_project_structure/chapter.md#addons

This refers to examples/addons which is no longer existent in the 0.10 OF branch.

[tpltnt]

@frauzufall Did I miss any examples in #182?