fix destructuring errors in esbuild example in preprocessing.md

[SpicyRicecaker]

In the preprocessing.md doc there seems to be a destructuring error.

const { js: code } //... js doesn't exist
return { code };

Using the code above makes it so that the compiler throws errors. The latest esbuild transform api doesn't return an object with the js property but instead has the code property.

The small changes below fixes the error and actually mirrors some of the code in the custom preprocessors section slightly above the esbuild example.

const { code, map } = //...

[kaisermann]

Nice! Thanks 🙏