fix callback_sink example, preventing unused parameter error

example/example.cpp

@@ -142,7 +142,7 @@ void daily_example()
 void callback_example()
 {
     // Create the logger
-    auto logger = spdlog::callback_logger_mt("custom_callback_logger", [](const spdlog::details::log_msg &msg) {
+    auto logger = spdlog::callback_logger_mt("custom_callback_logger", [](const spdlog::details::log_msg &/*msg*/) {
         // do what you need to do with msg
     });
 }