Simplify syntax in nbconvert-css webpack config

packages/nbconvert-css/webpack.config.js

@@ -16,12 +16,7 @@ module.exports = {
     rules: [
       {
         test: /\.css$/,
-        use: [
-          {
-            loader: MiniCssExtractPlugin.loader
-          },
-          'css-loader'
-        ]
+        use: [MiniCssExtractPlugin.loader, 'css-loader']
       },
       {
         test: /\.svg(\?v=\d+\.\d+\.\d+)?$/,