Fix the sign of the quadratic term in ArtificialViscosityMonaghan

[efaulhaber]

Based on #1086.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.48%. Comparing base (a720b1d) to head (1de6e98).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...chemes/structure/total_lagrangian_sph/viscosity.jl	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (a720b1d) and HEAD (1de6e98). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (a720b1d)	HEAD (1de6e98)
total	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1295       +/-   ##
===========================================
- Coverage   90.39%   77.48%   -12.91%     
===========================================
  Files         138      138               
  Lines       11322    11299       -23     
===========================================
- Hits        10234     8755     -1479     
- Misses       1088     2544     +1456     

Flag	Coverage Δ
total	?
unit	77.48% <50.00%> (ø)

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