fix log line typo
fix log line typo

log lines didn't make sense because the arguments were the wrong way
round.

As an experiment to try and clean up some of our code a bit, this commit
adds f-strings. f-strings were added in python 3.6, as a way to clean
up, simplify, and improve the performance of `str.format`.

alphagov/notifications-api,alphagov/notifications-api