Fix reference to Patterns and Matching chapter using the wrong chapter number

[KrappRamiro]

Hi.

Im new to Rust, and i saw chapter 2 of the book saying the following:

Patterns and the match construct are powerful Rust features: they let you express a variety of situations your code might encounter and they make sure you handle them all. These features will be covered in detail in Chapter 6 and Chapter 18, respectively.

But when i look at the index, i see that Chapter 18 is about OOP, and the one about Patterns and Matching is instead Chapter 19, so i figured that it may be an error