Fixed DockerFile build warning (ros-navigation#4462)

Signed-off-by: Jatin Patil <jatinpatil2003@gmail.com>

Dockerfile

@@ -168,7 +168,7 @@ RUN mkdir -p $ROOT_SRV
 
 # install demo dependencies
 RUN apt-get update && apt-get install -y \
- ros-$ROS_DISTRO-rviz2 \
+ ros-$ROS_DISTRO-rviz2 
 
 # install gzweb dependacies
 RUN apt-get install -y --no-install-recommends \