message: dnn: fix typo in object_detection.cpp sample
diff: @@ -22,7 +22,7 @@ const char* keys =
  "{ height | -1 | Preprocess input image by resizing to a specific height. }"
  "{ rgb | | Indicate that model works with RGB input images instead BGR ones. }"
  "{ thr | .5 | Confidence threshold. }"
  - "{ thr | .4 | Non-maximum suppression threshold. }"
  + "{ nms | .4 | Non-maximum suppression threshold. }"
  "{ backend | 0 | Choose one of computation backends: "
  "0: automatically (by default), "
  "1: Halide language (http://halide-lang.org/), "
  