fix: minor typo in doc comments: of -> off

[heckj]

minor typo fix

[codecov]

Codecov Report

Merging #7 (10f73df) into main (604410b) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #7   +/-   ##
=======================================
  Coverage   98.10%   98.10%           
=======================================
  Files           4        4           
  Lines        1631     1631           
=======================================
  Hits         1600     1600           
  Misses         31       31           

Impacted Files	Coverage Δ
Sources/Histogram/Histogram.swift	96.08% <ø> (ø)

Impacted Files	Coverage Δ
Sources/Histogram/Histogram.swift	96.08% <ø> (ø)

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[dimlio]

Thanks!