Fix typo in Webpack 4 configuration

Fixed a missing single quote at the beginning of the string in the Webpack 4 configuration example.

docs/documentation/customize/with-webpack.html

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