Minor correction and suggestion for chapter 9

[TomaszZamacinski]

There is an issue on chapter 9, in the following paragraph:

We also rewrite potential outcomes with a double indexing notation, where the first index denotes the counterfactual for the instrument and the second index, for the treatment

It should be:

We also rewrite potential outcomes with a double indexing notation, where the first index denotes the counterfactual for the treatment and the second index, for the instrument

On a different note, since IV2SLS.from_formula is being used for OLS estimation in this chapter I think it would be beneficial to explicitly state that OLS is a special case of IV2SLS.