doc: future-util: fix doxygen group
doc: future-util: fix doxygen group

It should be nested properly within the namespace block.

resolves the following doxygen warning:
Parsing file .../include/seastar/core/future-util.hh:1472: warning: unbalanced grouping commands

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

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