[docs] fix csv encoding option

[ujiuji1259]

#317 added the encoding option into CsvFileProcessor, but the following code in the docs doesn't work.

def output(self):
    return self.make_target('file_name.csv', encoding='cp932')

In fact, we have to directly pass CsvFileProcessor with encoding='cp932' to TaskOnKart.make_target.

[Hi-king]

@ujiuji1259 THX!