DOC: Fix Driscoll-Kraay covariance index

[nyxst4ck]

Summary

• replace the fixed Gamma_1 term in the Driscoll-Kraay HAC sum with the indexed Gamma_i term
• keep the LyX source and included text representation aligned

Closes #583

Validation

• the changed math directive parses with Docutils
• git diff --check passes
• the full Sphinx build could not start locally because the editable native package build requires the Visual Studio toolchain

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (b535f55) to head (7dffb23).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #704   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files         101      101           
  Lines       17426    17426           
  Branches     1430     1430           
=======================================
  Hits        17347    17347           
  Misses         29       29           
  Partials       50       50           

Flag	Coverage Δ
adder	99.52% <ø> (ø)
subtractor	99.52% <ø> (ø)

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