fix README.md

[arnoldczhang]

missing )

[codecov]

Codecov Report

Merging #333 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #333   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         148    148           
  Lines        2600   2600           
  Branches      434    434           
=====================================
  Hits         2600   2600

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