[TMVA] ROOT-9495 -- Fix BDTG regression after parallelisation
[TMVA] ROOT-9495 -- Fix BDTG regression after parallelisation

Typo, should have been looping over fEventSample instead of local
variable eventSample.

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