Update URDF Tutorial for ROS 2

[mohseninima]

I noticed that the tutorial for URDFs is for ROS 1 and has not been updated for ROS 2. It doesn't explicitly say which version of ROS is needed, so it might be confusing to new users.

http://gazebosim.org/tutorials?tut=ros_urdf&cat=connect_ros

[Acwok]

I also spent hours to find out the tag naming convention was changed...
The Tutorial: Using Gazebo plugins with ROS is outdated. It even refers to a libgazebo_ros_multicamera.so for multicamera/stereobench which is now included in the libgazebo_ros_camera.so.

[Jan-Franck]

I had similar issues.
I wanted to use libgazebo_ros_laser.so and had to spend hours of debugging and research to find out, that it is now integrated in libgazebo_ros_ray_sensor.so for ROS2.
So I think the tutorial page for the plugins needs an update or it should at least be mentioned that there are differences between the ROS and ROS2 packages.