Use critical service deregistration key as provided in Consul docs

It's DeregisterCriticalServiceAfter, not Deregister_critical_service_after.
This's been working for a long time, mostly likely because Consul
converts snake_case keys to CamelCase internally but this is unverified.

src/autocluster_consul.erl

@@ -523,7 +523,7 @@ registration_body_maybe_add_deregister(Payload) ->
         -> list().
 registration_body_maybe_add_deregister(Payload, undefined) -> Payload;
 registration_body_maybe_add_deregister(Payload, Deregister_After) ->
-    Deregister = {'Deregister_critical_service_after',
+    Deregister = {'DeregisterCriticalServiceAfter',
         list_to_atom(service_ttl(Deregister_After))},
     Payload ++ [Deregister].
 %%--------------------------------------------------------------------

test/src/autocluster_consul_tests.erl

@@ -52,7 +52,7 @@ build_registration_body_test_() ->
             [{'Notes','RabbitMQ Auto-Cluster Plugin TTL Check'},
               {'TTL','30s'},
               {'Status', 'passing'},
-              {'Deregister_critical_service_after','257s'}]}],
+              {'DeregisterCriticalServiceAfter','257s'}]}],
         ?assertEqual(Expectation, autocluster_consul:build_registration_body())
       end},
       {"with unset deregister and ttl", fun() ->