docs: typo in README code example (#664)

Missing comma after the commented line

Author: fouronnes

README.md

@@ -702,7 +702,7 @@ module.exports = {
   optimization: {
     minimizer: [
       // For webpack@5 you can use the `...` syntax to extend existing minimizers (i.e. `terser-webpack-plugin`), uncomment the next line
-      // `...`
+      // `...`,
       new CssMinimizerPlugin(),
     ],
   },