docs: uglify -> terser (#350)

README.md

@@ -121,17 +121,13 @@ While webpack 5 is likely to come with a CSS minimizer built-in, with webpack 4
 **webpack.config.js**
 
 ```js
-const UglifyJsPlugin = require("uglifyjs-webpack-plugin");
+const TerserJSPlugin = require("terser-webpack-plugin");
 const MiniCssExtractPlugin = require("mini-css-extract-plugin");
 const OptimizeCSSAssetsPlugin = require("optimize-css-assets-webpack-plugin");
 module.exports = {
   optimization: {
     minimizer: [
-      new UglifyJsPlugin({
-        cache: true,
-        parallel: true,
-        sourceMap: true // set to true if you want JS source maps
-      }),
+      new TerserJSPlugin({}),
       new OptimizeCSSAssetsPlugin({})
     ]
   },