Updating test to use defined var

Author: annahay4

rust-runtime/aws-smithy-runtime/src/client/retries/token_bucket.rs

@@ -487,7 +487,7 @@ mod tests {
         let mut handles = Vec::new();
 
         // Launch threads that all start simultaneously
-        for _ in 0..10 {
+        for i in 0..10 {
             let atomic_clone = Arc::clone(&atomic);
             let barrier_clone = Arc::clone(&barrier);
             let handle = thread::spawn(move || {