Rename the "killing of Brian Thompson" into more proper title

BUR #35055 is pending approval.

rename killing_of_brian_thompson -> assassination_of_brian_thompson

I know the memes and things surrounding this, but I think this should at least got a proper tag wording in line with other similar tags like 2024 attempted assassination of Donald Trump.

I think we should just go with whatever Wikipedia calls it. It's currently named Killing of Brian Thompson there, so I think it should stay as it is.