[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.

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

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

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Checklist

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

[EmmaRenauld]

Thanks!

I didn't check: would it be possible to change a test so that we get this?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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	Coverage Δ
Scripts	69.25% <100.00%> (+0.03%)	⬆️
Library	66.86% <ø> (ø)

[EmmaRenauld]

P.s. Numero 1000!!!!