sdf_worlds.md refers to an incomplete world_demo.sdf

[egreglee]

Environment

• OS Version: Ubuntu 24.04
• Binary version 8.9.0

Description

• Expected behavior: the tutorial SDF worlds says "You can also download the finished world for this tutorial from here".
• Actual behavior: the sdf at world_demo.sdf lacks many of the sections described in the tutorial, i.e. it's not "finished".