Clarify ZeroSumNormal covariance documentation

[swarnaliiiiii]

Fixes: #7904

This PR clarifies the documentation for ZeroSumNormal to use K (size of the constrained axis)
instead of n (number of zero-sum axes), as discussed in the issue.

It updates the docstring math expression:

ZSN(σ) = N(0, σ² (I_K - 1/K J_K))

No functional code changes — documentation only.

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[ricardoV94]

Should not remove any of this previous stuff

[swarnaliiiiii]

Updated the docstring to use K (size of the constrained axis) instead of n in the covariance formula. This corrects the notation as discussed in issue #7904. No other text was removed or changed.

[ricardoV94]

There are clearly other changes, which you can confirm by clicking on

[swarnaliiiiii]

The math block has been updated to use K (size of the constrained axis) instead of n, with all previous documentation left intact. Only a part of the formula was replaced.

[lucianopaz]

Lgtm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (ebd836e) to head (73f423d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7988   +/-   ##
=======================================
  Coverage   91.49%   91.49%           
=======================================
  Files         116      116           
  Lines       18972    18972           
=======================================
  Hits        17359    17359           
  Misses       1613     1613           

Files with missing lines	Coverage Δ
pymc/distributions/multivariate.py	93.91% <ø> (ø)

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