Fix Linter API example

[regseb]

Example of Linter API in README is incorrect:

const linter = linter.createInstance({
      ^

SyntaxError: Identifier 'linter' has already been declared

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (51af13f) to head (437e5e9).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5864   +/-   ##
=======================================
  Coverage   98.78%   98.78%           
=======================================
  Files          51       51           
  Lines        2892     2892           
  Branches      888      888           
=======================================
  Hits         2857     2857           
  Misses         35       35           

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[willdurand]

Thanks!