fix typo

[KeremKurban]

Fixes Small Typo in the documentation.

Description

Improving documentation on https://morphoclass.readthedocs.io/en/latest/sections/using_gnn.html

How to test?

No test needed

Checklist

• [ ] This PR refers to an issue from the issue tracker. (if it is not the case, please create an issue first).
• [ ] Unit tests added. (if needed)
• [ ] setup.cfg, requirements.txt, and constraints.txt updated with new dependencies. (if needed)
• [ ] Type annotations added. (if a function is added or modified)
• [ ] All CI tests pass.