a possible bug in the video recording

https://github.com/semitable/fast-marl/blob/0711d925e816fa49f252702f17b8b766590321e7/fastmarl/ac/train.py#L76

I think second component of the if statement is either integer % bool.
In order to record the video with different frequencies we may need to do `step % frequency == 0` instead.  