[GUI_tutorial.md] Change `--force-version` value in Harmonic tutorial to a supported value.

[mmmarinho]

📜 Documentation Fix

Replace

gz sim --force-version 8.0.0 shapes.sdf

by

gz sim --force-version 8 shapes.sdf

in the harmonic docs.

Summary

The command below, in the harmonic harmonic docs,

gz sim --force-version 8.0.0 shapes.sdf

outputs the following error in modern versions (such as 8.9.0 and 8.10.0)

Version error: I cannot find this command in version [8.0.0]

Checklist

• [x] Signed all commits for DCO
• [x] Updated documentation (as needed)