Update thread_pool.h

include/spdlog/details/thread_pool.h

@@ -88,7 +88,7 @@ public:
     thread_pool(size_t q_max_items, size_t threads_n, std::function<void()> on_thread_start);
     thread_pool(size_t q_max_items, size_t threads_n);
 
-    // message all threads to terminate and join them
+    // message all threads to terminate gracefully and join them
     ~thread_pool();
 
     thread_pool(const thread_pool &) = delete;