Change example with race conditions

FAQ.md

@@ -210,16 +210,19 @@ way, when you are searching down some path that's already longer than
 this shortest route, you can just stop and avoid wasted effort. In
 sequential land, you might model this "best result" as a shared value
 like `Rc<Cell<usize>>` (here the `usize` represents the length of best
-path found so far); in parallel land, you'd use a `Arc<AtomicUsize>`.
+path found so far); in parallel land, you'd use a `&AtomicUsize`.
+Note that `Arc` is no longer necessary because atomic types provide interior mutability.
 Now we can make our search function look like:
 
 ```rust
-fn search(path: &Path, cost_so_far: usize, best_cost: &Arc<AtomicUsize>) {
+fn search(path: &Path, cost_so_far: usize, best_cost: &AtomicUsize) {
     if cost_so_far >= best_cost.load(Ordering::SeqCst) {
         return;
     }
     ...
-    best_cost.store(...);
+    // Note: doing `best_cost.store(...)` will introduce race conditions.
+    // Using `fetch_*` such as `fetch_min` will help prevent race conditions.
+    best_cost.fetch_min(cost_so_far, Ordering::SeqCst);
 }
 ```