docs: Remove cacheDirectory from babelLoader config

I removed that in #383 but it was skipped somehow. 
We discussed about this and this option doesn't seems to work with current babel-loader version (cf #383 (comment))

Ref #382 (comment)

Author: MoOx

packages/website/docs/learn/environment-setup/02-next.md

@@ -26,7 +26,6 @@ Babel is not the default compiler when using Next.js App Router, but it can stil
 const dev = process.env.NODE_ENV !== 'production'
 
 const config = {
-  cacheDirectory: true,
   parserOpts: {
     plugins: ['typescript', 'jsx'],
   },