Fix documentation for k in s2_cell_child()

[paleolimbot]

Closes #225.

I opted to change the documentation: everywhere else we pass values directly to S2 (e.g., level) even though those are 0-indexed, too. This also has the benefit of not breaking any existing usage.

[codecov-commenter]

Codecov Report

Merging #230 (4cf665f) into main (90ae67a) will not change coverage.
The diff coverage is n/a.

❗ Current head 4cf665f differs from pull request most recent head 44923ac. Consider uploading reports for the commit 44923ac to get more accurate results

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@@           Coverage Diff           @@
##             main     #230   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files          46       46           
  Lines        3517     3517           
=======================================
  Hits         3307     3307           
  Misses        210      210           

Impacted Files	Coverage Δ
R/s2-bounds.R	100.00% <ø> (ø)
R/s2-cell.R	100.00% <ø> (ø)

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