Change tutorial to use `fatfree-core` instead of `fatfree`

When changing in the tutorial from `fatfree` to `fatfree-core` we are loosing the `lib` folder. The tutorial explains that we can use it to add additional libraries there. Since most projects are using Composer anyway, this is no loss IMHO. It would be nice to mention that it it's easy to configure the built-in autoloader or to inform the reader about the autoloading section.