Question: when was the last time celtics won nba championship
[{'answer': '2008', 'provenance': [{'wikipedia_id': '43376', 'title': 'Boston Celtics', 'start_paragraph_id': 2, 'start_character': 310, 'end_paragraph_id': 2, 'end_character': 314, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]