PyNumero: Fix typo in docstring

[blnicho]

Summary/Motivation:

Fix a small typo in a PyNumero docstring

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[mrmundt]

Too bad there isn't an action for grammar.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (6594436) 87.49% compared to head (5938258) 87.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2873      +/-   ##
==========================================
- Coverage   87.49%   87.48%   -0.02%     
==========================================
  Files         771      771              
  Lines       89598    89598              
==========================================
- Hits        78396    78387       -9     
- Misses      11202    11211       +9     

Flag	Coverage Δ
linux	84.55% <ø> (-0.01%)	⬇️
osx	74.60% <ø> (ø)
other	84.72% <ø> (ø)
win	82.12% <ø> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...o/contrib/pynumero/interfaces/external_grey_box.py	88.88% <ø> (ø)

... and 2 files with indirect coverage changes

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