Error in warning issued when enforcing LAL kwargs

[jacobgolomb]

The warning introduced by this MR: https://git.ligo.org/lscsoft/bilby/-/merge_requests/1269 is printed by this line https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/gw/source.py?ref_type=heads#L672.

This raises a KeyError because I don't think you can print a dict with the .format method like this. I think it needs to be changed to .format(waveform_kwargs=waveform_kwargs).