Fix typo in README.md

[spinza]

Pre-Request Checklist

• [n/a] Passes rubocop code analysis (try rubocop --auto-correct)
• [n/a] Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• [n/a] User-visible changes appended to CHANGELOG.md

Description

Fix small typo in README.

[codecov-io]

Codecov Report

Merging #1648 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1648     +/-   ##
========================================
+ Coverage   62.49%   62.6%   +0.1%     
========================================
  Files          30      30             
  Lines        1493    1492      -1     
========================================
+ Hits          933     934      +1     
+ Misses        560     558      -2

Impacted Files	Coverage Δ
lib/oxidized/node.rb	71.32% <0%> (-0.2%)	⬇️
lib/oxidized/output/git.rb	18.25% <0%> (+1.58%)	⬆️

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