Confirm that stationarized foreign debt law of motion is incorrect in documentation

[rickecon]

@jdebacker. I wanted to confirm that the partially open economy stationarized foreign debt holdings law of motion in the Foreign Debt Purchases section of the Open Economy Options chapter of the current documentation is incorrect. It is currently displayed as the following.

However, my math shows that it should be the following.

The only difference is that I think your g_{n,t+1} should be changed to 1 + \tilde{g}_{n,t+1}. I find that this value in the code new_borrowing_f is specified in the way I describe above and not the way that is currently shown in the documentation.

[jdebacker]

@rickecon I agree - typos in the docs.

[rickecon]

@jdebacker . OK. I have fixed this in open PR #683.