Bollinger Bands

I propose this change
`    def bollinger_bands(self, df, n):
        """
        :param df: pandas.DataFrame
        :param n: 
        :return: pandas.DataFrame
        """
        sma = data.rolling(window = n).mean()    
        std = data.rolling(window = n).std()
        upper_bb = sma + std * 2
        lower_bb = sma - std * 2
        return upper_bb, lower_bb
`