adding the print age line

lectures/27_garbage_collection/README.md

@@ -369,5 +369,10 @@ int main(){
         // the shared_ptr itself is now empty (i.e., it holds nullptr).
         age.reset();
         std::cout << "the use count is : " << age.use_count() << std::endl;
+
+        // question: what happens if we print age here:
+        // std::cout << "age is " << *age << std::endl;
+
+        return 0;
 }
 ```