Change tutorial to use fatfree-core instead of fatfree

[Rayne]

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.