Update Policy creation 'towhat' field documentation

Original bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1659899

Messaging bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1663562

With the above bug, it was determined that the documentation - which
was used to test the Policy create action in the API - for the `towhat`
value was not reflective of a valid value for that field.

This change updates the documentation to reflect recent changes which
added an inclusion whitelist for `towhat` values on creation of Policies.

Relevant PRs:
Inclusion list: https://github.com/ManageIQ/manageiq/pull/18032/files
Error messaging: ManageIQ/manageiq#18338

api/reference/policies.adoc

@@ -216,7 +216,7 @@ POST /api/policies
   "name" : "sample_policy",
   "description" : "Sample Policy",
   "mode" : "compliance",
-  "towhat" : "ManageIQ::Providers::Redhat::InfraManager",
+  "towhat" : "Vm",
   "condition_ids" : [11, 12],
   "policy_contents" : [
     {