[Doc] Fix a broken link in README

[sinsoku]

I noticed that the Contributor Covenant badge is a broken link, so I fix it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (0c6f23f) to head (320cb00).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #385   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files          14       14           
  Lines         574      574           
  Branches      139      139           
=======================================
  Hits          573      573           
  Misses          1        1           

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[okuramasafumi]

Good catch! Thanks!
CI is failing but it's my mistake, I'll fix it soon.