Fix typo and be clearer about ecma settings in uglifyjs options (#4239)

Author: danielberndt

packages/react-scripts/config/webpack.config.prod.js

@@ -379,9 +379,10 @@ module.exports = {
     new UglifyJsPlugin({
       uglifyOptions: {
         parse: {
-          // we want uglify-js to parse ecma 8 code. However we want it to output
-          // ecma 5 compliant code, to avoid issues with older browsers, this is
-          // whey we put `ecma: 5` to the compress and output section
+          // we want uglify-js to parse ecma 8 code. However, we don't want it
+          // to apply any minfication steps that turns valid ecma 5 code
+          // into invalid ecma 5 code. This is why the 'compress' and 'output'
+          // sections only apply transformations that are ecma 5 safe
           // https://github.com/facebook/create-react-app/pull/4234
           ecma: 8,
         },