docs: fix simple example (webpack#189)

Author: gerasimov

README.md

@@ -58,8 +58,8 @@ module.exports = {
       {
         test: /\.css$/,
         use: [
-          { 
-            MiniCssExtractPlugin.loader,
+          {
+            loader: MiniCssExtractPlugin.loader,
             options: {
               // you can specify a publicPath here
               // by default it use publicPath in webpackOptions.output