Update wording in README

[Jiahao01121]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

The instruction should address copy-webpack-plugin as a plugin, as opposed to a loader.

Breaking Changes

None.

Additional Info

[jsf-clabot]

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[codecov]

Codecov Report

Merging #386 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #386   +/-   ##
=======================================
  Coverage   94.82%   94.82%           
=======================================
  Files           9        9           
  Lines         290      290           
  Branches       78       78           
=======================================
  Hits          275      275           
  Misses         14       14           
  Partials        1        1

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[alexander-akait]

Thanks