doc: shared_future: fix doxygen group
doc: shared_future: fix doxygen group

It should be nested properly within the namespace block.

Resolves the following doxygen warning:
Parsing file .../include/seastar/core/shared_future.hh:299: warning: end of file while inside a group

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

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