fix: update threads2.rs to use Mutex

exercises/threads/threads2.rs

@@ -5,7 +5,7 @@
 
 // I AM NOT DONE
 
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
@@ -14,7 +14,7 @@ struct JobStatus {
 }
 
 fn main() {
-    let status = Arc::new(JobStatus { jobs_completed: 0 });
+    let status = Arc::new(Mutex::new(JobStatus { jobs_completed: 0 }));
     let mut handles = vec![];
     for _ in 0..10 {
         let status_shared = Arc::clone(&status);