lisa.trace: Rename Trace.df_events() into df_event()

Since that function only gives the dataframe of one event at a time,
remove the "s" from its name.

Note: this commit was 100% generated using the following command:
sed --follow-symlinks -i 's/df_events/df_event/g' {lisa,doc}/**.py doc/**.rst ipynb/**ipynb tools/*.py