README: Fix build mode documentation
README: Fix build mode documentation

Ensure that the commands can actually be executed.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20190114233207.16867-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

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