Add example of struct update syntax

struct_example/src/main.rs

@@ -18,4 +18,12 @@ fn build_user(email: String, username: String) -> User {
 fn main() {
     let user = build_user(String::from("Heisenberg"), String::from("mail@mail.com"));
     println!("Hello to the User: {:?}!", user);
+
+    let user2 = User {
+        username: String::from("Jesse"),
+        email: String::from("mail@mail.com"),
+        ..user
+    };
+
+    println!("Hello to the User: {:?}!", user2);
 }