message: Fix no_cluster docs
  By default, no_cluster returns 503 not a 404.
diff: @@ -371,7 +371,7 @@ owning HTTP connection manager.
  :widths: 1, 1, 2
  no_route, Counter, Total requests that had no route and resulted in a 404
  - no_cluster, Counter, Total requests in which the target cluster did not exist and resulted in a 404
  + no_cluster, Counter, Total requests in which the target cluster did not exist and which by default result in a 503
  rq_redirect, Counter, Total requests that resulted in a redirect response
  rq_direct_response, Counter, Total requests that resulted in a direct response
  rq_total, Counter, Total routed requests
  