Fix the name of an autoscaling policy test

The test name says it is testing the put autoscaling decision API, but
that is not right, since no such API exists (nor will exist). This
commit corrects the name of this test to reflect the fact that the test
is about the put autoscaling policy API.

x-pack/plugin/autoscaling/qa/rest/src/test/resources/rest-api-spec/test/autoscaling/put_autoscaling_policy.yml

@@ -1,5 +1,5 @@
 ---
-"Test put autoscaling decision":
+"Test put autoscaling policy":
   - do:
       autoscaling.put_autoscaling_policy:
         name: hot