Fix CSS filename for Webpack 4 config

docs/documentation/customize/with-webpack.html

@@ -116,7 +116,7 @@ module.exports = {
   },
   plugins: [
     new MiniCssExtractPlugin({
-      filename: 'css/[name].bundle.css'
+      filename: 'css/mystyles.css'
     }),
   ]
 };