Fix command reference in error message

[zinelya]

Quick description

removing ".py" from
To convert your data, you may use tools like mrconvert or >> scil_volume_math.py convert IMG IMG --data_type uint8 -f

Type of change

Check the relevant options.

• 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)

...

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.91%. Comparing base (750d299) to head (bbb0697).
⚠️ Report is 2 commits behind head on master.

❌ Your project status has failed because the head coverage (72.91%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1314   +/-   ##
=======================================
  Coverage   72.91%   72.91%           
=======================================
  Files         295      295           
  Lines       25453    25453           
  Branches     3567     3567           
=======================================
  Hits        18560    18560           
+ Misses       5389     5388    -1     
- Partials     1504     1505    +1     

Flag	Coverage Δ
smoketests	69.75% <ø> (ø)
unittests	14.28% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Scripts	75.44% <ø> (ø)
Library	69.90% <ø> (ø)

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