+Doc #20192 update http example to always consume response (#21241)

The single-request-in-actor-example does not consume the response
in where the status code is not OK.  Although this is unlikely
to cause backpressure issues as the response is likely to have
a HttpEntity.Strict entity, the example is confusing as it
does not follow the warning about always consuming the response.

akka-docs/rst/scala/code/docs/http/scaladsl/HttpClientExampleSpec.scala

@@ -207,8 +207,9 @@ class HttpClientExampleSpec extends WordSpec with Matchers with CompileOnlySpec
       def receive = {
         case HttpResponse(StatusCodes.OK, headers, entity, _) =>
           log.info("Got response, body: " + entity.dataBytes.runFold(ByteString(""))(_ ++ _))
-        case HttpResponse(code, _, _, _) =>
+        case resp @ HttpResponse(code, _, _, _) =>
           log.info("Request failed, response code: " + code)
+          resp.discardEntityBytes()
       }
 
     }