Change wrong usage in documentation about autoload_paths [ci skip]

[meinac]

We can't use this configuration outside of the application.rb

[kaspth]

Is that true, @fxn?

[meinac]

@kaspth you can try it by yourself. Just try to add new path to the autoload_paths in environment specific initialiser(it does not matter which environment you use) it will raise error while booting the application.

[zzak]

I believe this is because the paths get frozen after the app boots.

[meinac]

Yes, this is why we can't use it in environment specific initializer. Also freezing this makes sense because even if you change the autocad_paths it does not effect on the loaded paths.