Fix incorrect variables in documentation for Gazebo world and robot spawning

[redwine-1]

Updated paths for Gazebo world and robot spawning.

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Basic Info

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Ticket(s) this addresses	N/A
Does this PR contain AI-generated software?	No

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Description of contribution in a few bullet points

• Updated the documentation section titled “Add these two variables, which are needed for starting the Gazebo world and spawning the robot”.
• Previously, this section listed:

  default_model_path = os.path.join(pkg_share, 'src', 'description', 'sam_bot_description.sdf')
  default_rviz_config_path = os.path.join(pkg_share, 'rviz', 'config.rviz')

  However, these variables are not related to Gazebo world launching or robot spawning.
  They are already defined in an earlier section of the documentation.
• Replaced them with the correct variables actually used for Gazebo world and robot spawning:

  ros_gz_sim_share = get_package_share_directory('ros_gz_sim')
  gz_spawn_model_launch_source = os.path.join(ros_gz_sim_share, "launch", "gz_spawn_model.launch.py")

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Type of change

• Documentation fix
• New feature
• Bug fix
• Refactor

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[SteveMacenski]

Thanks!