Fix typo in README.md

[codecov-io]

Codecov Report

Merging #5 into master will decrease coverage by 1.27%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
- Coverage   77.16%   75.89%   -1.28%     
==========================================
  Files          19       19              
  Lines         784      784              
  Branches       91       91              
==========================================
- Hits          605      595      -10     
- Misses        179      189      +10

Impacted Files	Coverage Δ
...stdlib/src/main/scala/arrows/stdlib/ArrowRun.scala	80.55% <0%> (-12.5%)	:arrow_down:
...itter/src/main/scala/arrows/twitter/ArrowRun.scala	97.22% <0%> (-1.39%)	:arrow_down:

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[fwbrasil]

thank you!