Fix path to index.js in package.json

[onigoetz]

Hello @evilebottnawi I propose this change to the package.json as the file isn't in the src folder.
And this currently breaks css-loader when used with Yarn PNP.

[codecov-io]

Codecov Report

Merging #8 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files           2        2           
  Lines         243      243           
  Branches       71       71           
=======================================
  Hits          242      242           
  Misses          1        1

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[alexander-akait]

Done in https://github.com/css-modules/postcss-modules-local-by-default/releases/tag/v2.0.5, thanks!