tbl_survfit_times - add_difference_row should do the rate for Treatment Group – rate for Control Group

### Feature description

The `add_difference_row` function when used for class 'tbl_survfit_times' should do the rate for Treatment Group – rate for Control Group. At the moment it does this the other way around so say we expect to see a difference on -10.54, we see 10.54 instead. This also means the confidence intervals are centered around the incorrect point estimate. The GDSR states the following "For pairwise comparisons using the difference in event-free rates, the value for the control group is the subtrahend (i.e., rate for Treatment Group 1 – rate for Control Group)

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct.

### Contribution Guidelines

- [x] I agree to follow this project's Contribution Guidelines.

### Security Policy

- [x] I agree to follow this project's Security Policy.