Update global-variable.md(next_id取值错误？）

Atomic*::fetch_* 函数返回值为上一个值，next_id的取值是不是错了？

src/advance/global-variable.md

@@ -93,7 +93,8 @@ fn generate_id()->usize{
     if current_val > MAX_ID{
         panic!("Factory ids overflowed");
     }
-    let next_id = GLOBAL_ID_COUNTER.fetch_add(1, Ordering::Relaxed);
+    GLOBAL_ID_COUNTER.fetch_add(1, Ordering::Relaxed)
+    let next_id = GLOBAL_ID_COUNTER.load(Ordering::Relaxed);
     if next_id > MAX_ID{
         panic!("Factory ids overflowed");
     }