[bundle-analyzer] Fix incorrect example (#18952)

This confused me a lot, because in the case of `withBundleAnalyzer({})` it will ignore any previous config and basically override everything.

Furthermore, it makes more sense to move the call with `enabled` option inside so it's aligned with other plugins approach.

packages/next-bundle-analyzer/readme.md

@@ -51,8 +51,9 @@ const withPlugins = require('next-compose-plugins')
 const withBundleAnalyzer = require('@next/bundle-analyzer')({
   enabled: process.env.ANALYZE === 'true',
 })
+
 module.exports = withPlugins([
-  [withBundleAnalyzer({})],
+  [withBundleAnalyzer],
   // your other plugins here
 ])
 ```