Fix a typo in sentence_similarity.py

from SBERT_Model import SBERT_Model

from STSB_Dataset import STSB_Dataset

dataset = STSB_Dataset('data/stsbenchmark/sts-test.csv')._as_dataframe

dataset = STSB_Dataset('data/stsbenchmark/sts-test.csv').as_dataframe

zero_model = SBERT_Model("Zero Model", 'models/nli-bert-large/', dataset)

ft_model = SBERT_Model("Fine-tuned Model", 'models/stsb-bert-large/', dataset)