doc: Fix remove secret number at final source

Remove printing of secret number at final source.

Author: rydotyosh

src/doc/trpl/guessing-game.md

@@ -960,8 +960,6 @@ fn main() {
 
     let secret_number = rand::thread_rng().gen_range(1, 101);
 
-    println!("The secret number is: {}", secret_number);
-
     loop {
         println!("Please input your guess.");