Doc: remove obsolete example.
The documentation for ts_headline() recommends using a sub-select to
avoid extra evaluations of ts_headline() in a query with ORDER BY+LIMIT.
Since commit
9118d03a8 this contortionism is unnecessary, so remove the
recommendation. Noted by Oleg Bartunov.
Discussion: <CAF4Au4w6rrH_j1bvVhzpOsRiHCog7sGJ3LSX0tY8ZdwhHT88LQ@mail.gmail.com>