DOC: Fixed miniscule typo

[madphysicist]

I don't expect that this will be especially controversial. Ultimate test of the bikeshedding rule :)

[codecov]

Codecov Report

Merging #127 (e76e529) into master (57006e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #127   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files          10       10           
  Lines         460      460           
=======================================
  Hits          458      458           
  Misses          2        2           

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