fix typo in docstring

[chrishavlin]

calculation and variable names were correct, just mistyped the docstring

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (f9c8175) 95.98% compared to head (5dd4a68) 95.68%.
Report is 18 commits behind head on main.

Files	Patch %	Lines
aglio/_utilities/dependencies.py	84.61%	2 Missing ⚠️
aglio/seismology/collections.py	50.00%	2 Missing ⚠️
aglio/mapping.py	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   95.98%   95.68%   -0.31%     
==========================================
  Files          26       26              
  Lines        1369     1389      +20     
==========================================
+ Hits         1314     1329      +15     
- Misses         55       60       +5     

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