Fix field name for rendered DeploymentActions

In [code](https://github.com/mesosphere/marathon/blob/e775bee6cb67f66a9c8322d29693ef0e27460a8d/src/main/scala/mesosphere/marathon/api/v2/DeploymentsResource.scala#L66):
```scala
def actionToMap(action: DeploymentAction): Map[String, String] =
  Map(
    "action" -> action.getClass.getSimpleName,
    "apps" -> action.app.id.toString
  )
```

In [documentation](https://mesosphere.github.io/marathon/docs/rest-api.html#get-/v2/deployments) ([pinned](https://github.com/mesosphere/marathon/blob/31a4dca63861913ae9decb2e540c86f01574b84b/docs/docs/rest-api.md#get-/v2/deployments)):
```json
{
  "action": "ScaleApplication",
  "app": "/test"
}
```

It's a single ID, so I assume `app` was intended