correct the annotation of param

In the function trove.instance.service.py,
there is a function has four params,
the first three params's annotation is accurate，
but the last param is incorrect,
so I correct it for others

Change-Id: I1e910c5466861550704419d85d59d0da2e5b7b0f

trove/instance/service.py

@@ -67,7 +67,7 @@ class InstanceController(wsgi.Controller):
         :param req: http request object
         :param body: deserialized body of the request as a dict
         :param tenant_id: the tenant id for whom owns the instance
-        :param id: ???
+        :param id: instance id
         """
         LOG.debug("instance action req : '%s'\n\n", req)
         if not body: