message: Fix extra argument in readme.md
diff: @@ -123,7 +123,7 @@ To use `rtabmap_ros` on Jetson, you can follow the instructions above if you don
  $ cd ~/catkin_ws
  $ git clone https://github.com/ros-perception/vision_opencv src/vision_opencv
  $ git clone https://github.com/ros-perception/image_transport_plugins.git src/image_transport_plugins
  - $ git clone https://github.com/introlab/rtabmap_ros.git src/rtabmap_ros src/rtabmap_ros
  + $ git clone https://github.com/introlab/rtabmap_ros.git src/rtabmap_ros
  $ catkin_make -j2
  ```
  