doc: gate: fix \ref closed()
doc: gate: fix \ref closed()

Resolves following warning:
include/seastar/core/gate.hh:74: warning: unable to resolve reference to 'closed()' for \ref command

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar