modify gateway document

Author: shuqz

docs/guide/gateway/gateway.md

@@ -127,7 +127,7 @@ spec:
 
 When `my-http-service` or the configured service port can't be found,
 the target group will not be materialized on any ALBs that the route attaches to.
-An [503 Fixed Response](https://docs.aws.amazon.com/elasticloadbalancing/latest/APIReference/API_FixedResponseActionConfig.html)
+An [500 Fixed Response](https://docs.aws.amazon.com/elasticloadbalancing/latest/APIReference/API_FixedResponseActionConfig.html)
 will be added to any Listener Rules that would have referenced the invalid backend.
 
 ## Specify out-of-band Target Groups