[docs] Update franka_gazebo docs

[rickstaa]

You are probably aware of this issue, but I will add it since it is easily overlooked. The commands in the franka-gazebo are not working anymore because the panda namespace was removed by default.

rostopic pub --once /panda/franka_gripper/move/goal franka_gripper/MoveActionGoal "goal: {width: 0.08, speed: 0.1}"

should be

rostopic pub --once /franka_gripper/move/goal franka_gripper/MoveActionGoal "goal: {width: 0.08, speed: 0.1}"

Similar for the grasp command.

[gollth]

Thanks for the reminder @rickstaa. The fix is already incorporated in the docs internally, and will be published to https://frankaemika.github.io/ as soon as we release franka_ros#develop to melodic-devel/noetic-devel