[ntuple] Fix-up tutorial after RFieldBase::Create() interface change
[ntuple] Fix-up tutorial after RFieldBase::Create() interface change

root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,olifre/root,root-mirror/root