Rustlings threads2: Arc alone can't mutate, wrap in Mutex

In rustlings 20_threads/threads2, status_shared.jobs_done += 1 fails because Arc only gives shared & access. Fix is Arc<Mutex<JobStatus>>: add Mutex to the std::sync import, wrap the struct in Mutex::new, mutate via status_shared.lock().unwrap().jobs_done += 1, and read in main via status.lock().unwrap().jobs_done. Compiling the exercise directly with rustc --edition 2021 <file> is a quick sanity check outside the rustlings watcher.