fix: potential endless loop (#22)

* fix potential endless loop

* update code as well

book/zh-cn/07-thread.md

@@ -153,6 +153,7 @@ int main()
         cond_var.notify_one();
     }
     done = true;
+    notified = true;
     cond_var.notify_one();
 });
 

code/7/7.2.cpp

@@ -39,6 +39,7 @@ int main()
             cond_var.notify_one();
         }
         done = true;
+        notified = true;
         cond_var.notify_one();
     });
     // 消费者线程