docs(api_rest.md): JSON key of regress/classify response is results instead of result

[Ryuheeeei]

I thought the JSON key of regress and classify response is results, not result.

When I follow Example section of https://www.tensorflow.org/tfx/serving/api_rest, I got the following response which the key is results, not result as described in website. Also, I tried the classify request, and found it return the label/score pairs with the results key.

{
    "results": [3.5, 4.0]
}

It seems the response key is defined results here.

https://github.com/tensorflow/serving/blob/ebb71bfc52c4ac52f0061f356f9f81d919b5e841/tensorflow_serving/util/json_tensor.cc#L82

Sorry if I misunderstood. In case of this is my misunderstanding, I'll close this PR.