message: docs: fix header name for retries in gRPC services
diff: @@ -34,7 +34,8 @@ of requests are retried, how many times the request should be retried, timeouts
  Retries in gRPC services
  ------------------------
  -For gRPC services, Envoy looks at the gRPC status in the response and attempts a retry based on the statuses configured in *x-retry-grpc-on*.
  +For gRPC services, Envoy looks at the gRPC status in the response and attempts a retry based on the statuses configured in
  +:ref:`x-envoy-retry-grpc-on <config_http_filters_router_x-envoy-retry-grpc-on>`.
  The following application status codes in gRPC are considered safe for automatic retry.
  