Fix warn(unused_mut) in example

src/doc/book/patterns.md

@@ -303,7 +303,7 @@ struct Person {
 }
 
 let name = "Steve".to_string();
-let mut x: Option<Person> = Some(Person { name: Some(name) });
+let x: Option<Person> = Some(Person { name: Some(name) });
 match x {
     Some(Person { name: ref a @ Some(_), .. }) => println!("{:?}", a),
     _ => {}