Fix a python sample code

[abicky]

This pull request will fix the error like below:

$ python sample_train.py sample.model < input 
0.12.2
Traceback (most recent call last):
  File "sample_train.py", line 54, in <module>
    trainer.append(xseq, yseq, 0)
  File "/path/to/crfsuite/swig/python/crfsuite.py", line 130, in append
    def append(self, *args): return _crfsuite.Trainer_append(self, *args)
TypeError: in method 'Trainer_append', argument 3 of type 'CRFSuite::StringList const &'