Update README.md with a working example on data.TabularDataset for json files

[czhang99]

In README.md, the current example for data.TabularDataset with json file input looks like this:

sentiment = data.TabularDataset(
path='data/sentiment/train.json', format='json',
fields=[{'sentence_tokenized': ('text', data.Field(sequential=True)),
'sentiment_gold': ('labels', data.Field(sequential=False))}])

where "fields" attribute is a list that contains a dictionary. This will lead to an AttributeError: 'list' object has no attribute 'items' because in torchtext/data/example.py we have:

@classmethod
def fromJSON(cls, data, **fields**):
    return cls.fromdict(json.loads(data), **fields**)

@classmethod
def fromdict(cls, data, **fields**):
    ex = cls()
    for key, vals in **fields.items()**:

where the field value (currently a list) is processed by function fromdict with a ".item()" operator.

So the example in the README.md file should be changed to:

sentiment = data.TabularDataset(
path='data/sentiment/train.json', format='json',
fields={'sentence_tokenized': ('text', data.Field(sequential=True)),
'sentiment_gold': ('labels', data.Field(sequential=False))})

where "fields" attribute is fed with a dictionary directly.