Fix documentation on T2 measure

[franciscopana]

def t2(X, y):
    """
    Calculates the Average number of features per dimension (T2) metric. 

    To obtaint this measure, the number of dimensions describing the dataset is divided by the number of instances.

    .. math::

        T2=\\frac{m}{n}

    :type X: array-like, shape (n_samples, n_features)
    :param X: Dataset
    :type y: array-like, shape (n_samples)
    :param y: Labels

    :rtype: float
    :returns: T2 score
    """
    X = np.copy(X)
    return X.shape[1]/X.shape[0]

I think that the description of this measure is wrong. This measure, by diving number of features by the number of instances, calculates the Average Number of Features per Data Point. The description refers "Features per Dimension", which is not adequate considering the formula of this measure.