Revise citations in boussinesq.md

[navidcy]

Updated citations in the Boussinesq approximation section for clarity and consistency.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.71%. Comparing base (9584de0) to head (7f4de37).
⚠️ Report is 1 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (9584de0) and HEAD (7f4de37). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (9584de0)	HEAD (7f4de37)
	7	0
reactant_1	1	0
reactant_2	1	0
reactant_3	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5558      +/-   ##
==========================================
- Coverage   74.14%   68.71%   -5.43%     
==========================================
  Files         403      403              
  Lines       23333    22599     -734     
==========================================
- Hits        17300    15529    -1771     
- Misses       6033     7070    +1037     

Flag	Coverage Δ
buildkite	68.71% <ø> (-0.01%)	⬇️
julia	68.71% <ø> (-0.01%)	⬇️
reactant_1	?
reactant_2	?
reactant_3	?

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