Fix formatting in YOLO deserialize error message (#2535)

dlib/dnn/loss.h

@@ -3906,7 +3906,7 @@ namespace dlib
             if (count != tag_count())
                 throw serialization_error("Invalid number of detection tags " + std::to_string(count) +
                                           ", while deserializing dlib::loss_yolo_, expecting " +
-                                          std::to_string(tag_count()) + "tags instead.");
+                                          std::to_string(tag_count()) + " tags instead.");
             deserialize(item.options, in);
         }