Fix the handling of Rytov variance

[shunichironomura]

This PR:

• Fix the typo in function argument name
• Uses the Rytov variance provided as override in calculation

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.68%. Comparing base (2e8a41c) to head (cc5c231).

Files with missing lines	Patch %	Lines
src/cosmica/comm_link/uncertainty.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
+ Coverage   39.66%   39.68%   +0.02%     
==========================================
  Files          47       47              
  Lines        1926     1925       -1     
==========================================
  Hits          764      764              
+ Misses       1162     1161       -1     

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