1. It's == "it is"
2. The subject of the sentence is "Only one" (not "these") so the verb must agree with it.
Feel free to do nothing with this change, use it, etc - I just saw this while reading through the code.
// the active slide is heading.
// 2. .active.carousel-item is the current slide.
// 3. .active.carousel-item-left and .active.carousel-item-right is the current
-// slide in it's in-transition state. Only one of these occur at a time.
+// slide in its in-transition state. Only one of these occurs at a time.
// 4. .carousel-item-next.carousel-item-left and .carousel-item-prev.carousel-item-right
// is the upcoming slide in transition.