fix: typo in readthedocs-override example

[jtpio]

Description

Spotted while checking for a ReadTheDocs example.

How Has This Been Tested?

Checking against the ReadTheDocs schema.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

Thanks!