Update example_sklearn.py

[lenguyenthedat]

y must be a binary list, otherwise will result in an error:

[DEBUG] No existing model file or not options.incremental
[DEBUG] Running command: "vw --learning_rate=10.000000 --l2=0.000010 --loss_function=logistic --passes 10 --cache_file /Users/datle/Desktop/example_sklearn.cache -f /Users/datle/Desktop/example_sklearn.model"
[DEBUG] Running command: "vw --learning_rate=10.000000 --l2=0.000010 --loss_function=logistic -t -i /Users/datle/Desktop/example_sklearn.model -p /Users/datle/Desktop/example_sklearn.predictiond9d1DV"
Traceback (most recent call last):
  File "test.py", line 72, in <module>
    main()
  File "test.py", line 58, in main
    ).fit(X_train, y_train)
  File "/Library/Python/2.7/site-packages/sklearn/grid_search.py", line 732, in fit
    return self._fit(X, y, ParameterGrid(self.param_grid))
  File "/Library/Python/2.7/site-packages/sklearn/grid_search.py", line 505, in _fit
    for parameters in parameter_iterable
  File "/Library/Python/2.7/site-packages/sklearn/externals/joblib/parallel.py", line 659, in __call__
    self.dispatch(function, args, kwargs)
  File "/Library/Python/2.7/site-packages/sklearn/externals/joblib/parallel.py", line 406, in dispatch
    job = ImmediateApply(func, args, kwargs)
  File "/Library/Python/2.7/site-packages/sklearn/externals/joblib/parallel.py", line 140, in __init__
    self.results = func(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/sklearn/cross_validation.py", line 1478, in _fit_and_score
    test_score = _score(estimator, X_test, y_test, scorer)
  File "/Library/Python/2.7/site-packages/sklearn/cross_validation.py", line 1534, in _score
    score = scorer(estimator, X_test, y_test)
  File "/Library/Python/2.7/site-packages/sklearn/metrics/scorer.py", line 201, in _passthrough_scorer
    return estimator.score(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/sklearn/base.py", line 295, in score
    return accuracy_score(y, self.predict(X), sample_weight=sample_weight)
  File "/Library/Python/2.7/site-packages/sklearn/metrics/classification.py", line 179, in accuracy_score
    y_type, y_true, y_pred = _check_targets(y_true, y_pred)
  File "/Library/Python/2.7/site-packages/sklearn/metrics/classification.py", line 84, in _check_targets
    "".format(type_true, type_pred))
ValueError: Can't handle mix of binary and continuous