Fix attributes typo in docs

[RemDelaporteMathurin]

Proposed changes

Fix for #793

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [x] Documentation Update (if none of the other choices apply)
• [ ] New tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.53%. Comparing base (f1f9a71) to head (d2f54d1). Report is 156 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #795 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 59 59 Lines 2581 2581 ======================================= Hits 2569 2569 Misses 12 12 ```

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