Fix NameError in estimate_effects

[Will Fein]

Hello!

I believe that the variable name change introduced in this commit also introduced a bug. The variable was renamed from cv_folds to cf_folds but there are still two instances of cv_folds in estimate_effects. This is causing a NameError. I have also added a PR if you want to just change to the new name: #35

[Brian Quistorff]

Hey, thanks for reporting this! Your PR was missing another instance of the renamed variable so I pushed through my own commit.