Column name 'region' should be changed to 'data_type' in docs

[JohnNapier]

The data columns no longer contain 'region'. I believe documentation should be updated to reflect the change to 'data_type'.

[kwgoodman]

Numpy and pandas use the name data type to mean float64, int32, etc. So I picked the name region to represent train, validation, etc. Plus it is easier to type. But probably better to follow the naming that numerai uses.