Fix semantic mistake

The original phrase "As stated in the Gang of Four..." is semantically incorrect.
- The "Gang of Four" is a group of authors: Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides.
- They wrote a famous book: Design Patterns.
- Here, Ryan McDermott is referring to the software-design principle "you should prefer composition over inheritance."
- This principle was stated in the book, Design Patterns.
- It was not stated in the Gang of Four -- the authors themselves.