[ntuple, tutorial] fix bad assert expression
[ntuple, tutorial] fix bad assert expression

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