Fix a typo in docs/usage.md

[Changaco]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.51%. Comparing base (31936ab) to head (1fd21b9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files          11       11           
  Lines        1165     1165           
=======================================
  Hits         1136     1136           
  Misses         29       29           

Flag	Coverage Δ
unittests-3.10	97.51% <ø> (ø)
unittests-3.11	97.51% <ø> (ø)
unittests-3.12	97.51% <ø> (ø)
unittests-3.13	97.51% <ø> (ø)
unittests-3.9	97.51% <ø> (ø)

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