Correct typos in package index rule function doc comments

[per1234]

This was a copy/paste typo.

[codecov-commenter]

Codecov Report

Merging #195 (0e1ed24) into main (5ef2649) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #195   +/-   ##
=======================================
  Coverage   85.15%   85.15%           
=======================================
  Files          43       43           
  Lines        3389     3389           
=======================================
  Hits         2886     2886           
  Misses        392      392           
  Partials      111      111           

Flag	Coverage Δ
unit	85.15% <ø> (ø)

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Impacted Files	Coverage Δ
internal/rule/rulefunction/packageindex.go	89.92% <ø> (ø)

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