gh-125904: fix MRO summary discription in tutorial

[drewarnett]

The tutorial classes chapter multiple inheritance section described MRO as depth first. That was the case for the old classic classes in Python2.

• Issue: gh-125904

---

📚 Documentation preview 📚: https://cpython-previews--125906.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[drewarnett]

Cancelling this request. Leaving issue #125904 open to consider further how to update this section.