Fix typo in treebankPersister
Fix typo in treebankPersister

That one got missed during 6cb1060 in #159.

latin-language-toolkit/arethusa,fbaumgardt/arethusa,Masoumeh/arethusa,fbaumgardt/arethusa,alpheios-project/arethusa,latin-language-toolkit/arethusa,Masoumeh/arethusa,PonteIneptique/arethusa,fbaumgardt/arethusa,alpheios-project/arethusa,alpheios-project/arethusa,PonteIneptique/arethusa