rest_qos: fix a comment

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>
rest_qos: fix error of additional a qos rule with "priority"

Currently, rest_qos is impossible to set the "priority" of QoS rule on a switch.
Also, "priority" is should be described in the same depth as "match" and "actions".
This patch fixes these problems.

Now, rest_qos is possible to set the "priority" with the following curl command.

e.g.)
  $ curl -X POST -d '{"priority":"5",
                      "match": {"nw_dst": "10.0.0.1", "nw_proto": "UDP", "tp_dst": "5002"},
                      "actions":{"queue": "1"}}' http://localhost:8080/qos/rules/0000000000000001

Reported-by: Jace Liang <b436412@gmail.com>
Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>