[Typo] Tutorials -> Building 2D Games -> Chapter 12: Yellow Highlighting needed on Adding Collision To Our Game section

[jordanlhunt]

In Chapter 12 I noticed the yellow highlighting is missing. I did notice the overview of the changes at the end of the chapter of course, but I think yellow highlight is always useful for programmers to make sure they are making the correct changes. I have attached screenshots

[SimonDarksideJ]

Completed with #213