Fix haddocks for marker events in Debug.Trace
Right now, the haddocks for marker events are not displayed entirely: https://hackage.haskell.org/package/base-4.12.0.0/docs/Debug-Trace.html#g:3
The whole contents can be seen when looking at the source, right below traceEventIO
here, we're missing a few paragraphs, because of an empty line that should instead be --
.