Documentation errors on Rigid section

The current documentation has some errors in the Rigid section where degrees of freedom are mentioned. Several example xml entries feature both `dof` and `dofs`. It believe it should be the former. 

```xml
<dof>Rx</dofs>
```

<img width="1023" height="1019" alt="Image" src="https://github.com/user-attachments/assets/6c6445ed-2060-4210-8e29-f2eb5a31ddee" />