Changed to best practice of the this keyword

I've seen several people on IRC talk about readability. It seems there is some opinions that when chaining like this the `this` keyword is more readable then a dangling `@` symbol.

I think it is a good idea to offer code readability in this project.

Feel free to revert this commit if this is disputed. (Unable to open a PR from in the Github UI).

Author: sukima

chapters/classes_and_objects/chaining.md

@@ -20,13 +20,13 @@ class CoffeeCup
 			sugar: false
 	strength: (newStrength) ->
 		@properties.strength = newStrength
-		@
+		this
 	cream: (newCream) ->
 		@properties.cream = newCream
-		@
+		this
 	sugar: (newSugar) ->
 		@properties.sugar = newSugar
-		@
+		this
 
 morningCup = new CoffeeCup()