Update webpack.md

potcss-loader now uses the `postcssOptions` key. https://www.npmjs.com/package/postcss-loader

site/content/docs/5.0/getting-started/webpack.md

@@ -55,12 +55,14 @@ For Bootstrap to compile, make sure you install and use the required loaders: [s
   }, {
     loader: 'postcss-loader', // Run postcss actions
     options: {
+      postcssOptions: {
         plugins: function () { // post css plugins, can be exported to postcss.config.js
           return [
             require('autoprefixer')
           ];
         }
       }
+    }
   }, {
     loader: 'sass-loader' // compiles Sass to CSS
   }]