[FIX] Typo in scil_dti_metrics script

[AlexVCaron]

Quick description

scil_dti_metrics.py is unusable due to a typo in the script. I assume in the tests and most usages, we ask to output FA, and it must be why we didn't detect this error, since or shortcuts evaluating args.RGB.

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Type of change

Check the relevant options.

• [x] Bug fix (non-breaking change which fixes an issue)

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 68.27%. Comparing base (4e7047d) to head (0f4d6e0). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1000 +/- ## ========================================== + Coverage 68.24% 68.27% +0.02% ========================================== Files 419 419 Lines 21391 21391 Branches 3209 3209 ========================================== + Hits 14599 14604 +5 + Misses 5530 5528 -2 + Partials 1262 1259 -3 ``` | [Components](https://app.codecov.io/gh/scilus/scilpy/pull/1000/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/1000/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `69.25% <100.00%> (+0.03%)` | :arrow_up: | | [Library](https://app.codecov.io/gh/scilus/scilpy/pull/1000/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scilus) | `66.86% <ø> (ø)` | |

[EmmaRenauld]

P.s. Numero 1000!!!!