Update 2_5_id.cpp

主要是master_thread只进行了定义，没有赋值，进入main函数之后，才有master_thread_id; 其他就是加了一些输出，看起来直观一点

concurrency/concurrency_v1/chapter2/2_5_id.cpp

@@ -27,8 +27,12 @@ void some_core_part_of_algorithm() {
 }
 
 int main() {
-    std::cout << std::this_thread::get_id() << endl;
+    master_thread = std::this_thread::get_id();
+    std::cout << "master_thread: " << master_thread << endl;
+    cout << "master_thread 中运行:" << endl;
+    some_core_part_of_algorithm();
+    cout << "thread 中运行:" << endl;
     thread t(some_core_part_of_algorithm);
     t.join();
     return 0;
-}
+}