First I'd like to thank Publysher for the tutorial. It makes for an excellent Crash Course 3 and I really value the time you've put into this.
To offer my meager help for polishing this up, I'll list below the small ways of improvement that I noticed.
Part 1: "If the speed in the vertical direction (the y-direction) is negative, our actor is moving to the right." I believe the last word should be up, not right.
Part 2: I remember there was something small like in the first chapter, but have forgotten it while I worked my way through to chapter six and can't recall it anymore. I'll let you know if I remember it on a later reading.
Part 4: In "we can see a red dot and a green dot at screen coordinates (6,5) and (16,3)" the last coordinate should be (16,13) instead. "TIP: Every 2D game programmer should at least understand the concepts described above." I'd say understanding these at least on a surface level is most useful. The tip could indicate that it's nothing to be scared of if the reader didn't immediately feel like understanding all the concepts mentioned. It's typical for beginning programmers to learn these as they go.
Part 5: Under subtitle "Pick One", the first image could be better placed after the second paragraph instead of above the first.
The sentence coming after "TIP:" is sometimes written with a capital first letter, sometimes not. It would be the best to have the first letter always capitalized.
There also was an "is" that was written as "it" or the other way around on one chapter, but couldn't find it anymore afterwards.
I would have also welcomed a short explanation of why to use recycling for killing and creating bullets. On the later chapters the text sometimes omitted telling which attributes to create, but it was possible to figure everything out by the very helpful screenshots. Downloading the Action RPG tileset required registering on StencylForge, which some users might not want to do. While it's also good to familiarize the users with using StencylForge to download resources, providing an optional direct download link, if possible, would serve some readers best.
And finally, referring to the following screenshot, I personally changed the two occasions of "0 < 0" to "0 <= 0" when evaluating for Minimum Distance. This seemed to me more like the desired functionality and it makes the function behave usefully even when Minimum Distance is set to 0, returning true when the bullet is in the exact same spot as the enemy.

I hope this was helpful for Publysher and that this tutorial receives the official mantle of Crash Course 3. Can't wait for part 7!