Fix counter protobuf message codes

According to [this line](https://github.com/basho/riak_kv/blob/319e96717d5f4ee8afe6c47f65355f79035311f9/src/riak_kv_app.erl#L35) and my testing, these codes are the correct ones.

Is there a reason why they are not in the docs and `riak_pb` README?