reverting the hidden difficulty shift by 2 to make it consistent with the original source

src/game_server.rs

@@ -19,7 +19,7 @@ pub async fn handle(socket: &mut TcpStream) -> Result<(), String> {
     )
     .await?;
 
-    let depth = get_one_char(b"123456789", socket).await? - b'0' + 2;
+    let depth = get_one_char(b"123456789", socket).await? - b'0';
 
     let mut board = connect4::GameState::new();