tutorial: further fix service_loop example text
tutorial: further fix service_loop example text

Following up on 665ab5a99175517e0127b83c4ec89cf7129ed8e6

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20200719142943.67298-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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