doc fix: rename assim_mod_mod docs to match the module

[hkershaw-brown]

Description:

No rush on reviewing this, the rename has been hanging out for a while (#282).
Add this to the next documentation pull requests.

Fixes issue

fixes #282

Types of changes

• [ ] Bug fix (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)
• [x] Documentation update

Documentation changes needed?

• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.

cd DART
grep -rn assim_readme .
grep -rni assim_readme.html .

to see if any .rst files are using the filename as a link.

Checklist for merging

• [ ] Updated changelog entry
• [ ] Documentation updated
• [ ] Update conf.py

Checklist for release

• [ ] Merge into main
• [ ] Create release from the main branch with appropriate tag
• [ ] Delete feature-branch

Testing Datasets

• [ ] Dataset needed for testing available upon request
• [ ] Dataset download instructions included
• [x] No dataset needed