Fix example webpack config in documentation

docs/guides/getting-started.md

@@ -61,7 +61,7 @@ module.exports = {
     new webpack.DefinePlugin({
       'process.env.NODE_ENV': JSON.stringify('production')
     })
-  },
+  ],
 
   resolve: {
     // Maps the 'react-native' import to 'react-native-web'.