[FIX] fixed typo in surface convert script

[grahamlittlephd]

Quick description

Big start to scilpy mesh refactoring. Fixed a typo in convert surface script. Before fix script would only work with FreeSurfer surfaces.

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Type of change

Check the relevant options.

• [ x ] 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)
• [ ] This change requires a documentation update

Provide data, screenshots, command line to test (if relevant)

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Checklist

• [x ] My code follows the style guidelines of this project (run autopep8)
• [x ] I added relevant citations to scripts, modules and functions docstrings and descriptions
• [ x] I have performed a self-review of my code
• [ x] I have commented my code, particularly in hard-to-understand areas
• [ x] I have made corresponding changes to the documentation
• [ x] My changes generate no new warnings
• [ x] I moved all functions from the script file (except the argparser and main) to scilpy modules
• [ x] I have added tests that prove my fix is effective or that my feature works
• [ x] New and existing unit tests pass locally with my changes

[codecov[bot]]

Codecov Report

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

Project coverage is 67.97%. Comparing base (1ccddd8) to head (1d4a91b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #989 +/- ## ======================================= Coverage 67.97% 67.97% ======================================= Files 419 419 Lines 21615 21615 Branches 3251 3251 ======================================= Hits 14693 14693 Misses 5633 5633 Partials 1289 1289 ``` | [Components](https://app.codecov.io/gh/scilus/scilpy/pull/989/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | Coverage Δ | | |---|---|---| | [Scripts](https://app.codecov.io/gh/scilus/scilpy/pull/989/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `68.96% <100.00%> (ø)` | | | [Library](https://app.codecov.io/gh/scilus/scilpy/pull/989/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `66.49% <ø> (ø)` | |