[c++/r] Fix pkgdown build errors

[johnkerl]

Addresses https://github.com/single-cell-data/TileDB-SOMA/actions/runs/10900533993/job/30248339956

Since this only failed in the nightly pkgdown run, I manually ran that on this PR's branch:

• https://github.com/single-cell-data/TileDB-SOMA/actions/workflows/pkgdown.yaml
• https://github.com/single-cell-data/TileDB-SOMA/actions/runs/10904361981 -- ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.78%. Comparing base (1116d43) to head (8e6e14d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3003      +/-   ##
==========================================
+ Coverage   89.64%   89.78%   +0.14%     
==========================================
  Files          39       39              
  Lines        4094     4094              
==========================================
+ Hits         3670     3676       +6     
+ Misses        424      418       -6     

Flag	Coverage Δ
python	89.78% <ø> (+0.14%)	⬆️

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Components	Coverage Δ
python_api	89.78% <ø> (+0.14%)	⬆️
libtiledbsoma	∅ <ø> (∅)